diff --git a/mdx2/command_line/export.py b/mdx2/command_line/export.py
new file mode 100644
index 0000000..d56dc7f
--- /dev/null
+++ b/mdx2/command_line/export.py
@@ -0,0 +1,137 @@
+"""
+Convert merged HKL data from NeXus format to MTZ format.
+
+Miller indices are scaled by ndiv so that half-integer positions (ndiv=2)
+become integers in the output MTZ. The original unit cell is preserved.
+"""
+
+from dataclasses import dataclass
+from typing import Optional
+
+import numpy as np
+from simple_parsing import field
+
+from mdx2.command_line import make_argument_parser, with_logging, with_parsing
+from mdx2.io import loadobj
+
+
+@dataclass
+class Parameters:
+    """Options for converting merged NeXus data to MTZ format"""
+
+    hkl: str = field(positional=True)  # NeXus file containing merged hkl_table
+    geometry: str = field(positional=True)  # NeXus file containing crystal geometry
+    output: str = field(alias=["-o"], default="merged.mtz")  # output MTZ file
+    wavelength: float = 1.0  # wavelength in Angstroms
+    title: str = "mdx2 merged data"  # MTZ title
+
+def get_laue_it_number(it_number):
+    """Maps any Space Group IT number to its symmorphic Laue Group IT number."""
+    # This covers the most common cases (Triclinic through Cubic)
+    if 1 <= it_number <= 2: return 1    # -1
+    if 3 <= it_number <= 15: return 10  # 2/m
+    if 16 <= it_number <= 74: return 47 # mmm
+    if 75 <= it_number <= 88: return 83 # 4/m
+    if 89 <= it_number <= 142: return 123 # 4/mmm
+    if 143 <= it_number <= 148: return 147 # -3
+    if 149 <= it_number <= 167: return 162 # -3m
+    if 168 <= it_number <= 176: return 175 # 6/m
+    if 177 <= it_number <= 194: return 191 # 6/mmm
+    if 195 <= it_number <= 206: return 200 # m-3
+    if 207 <= it_number <= 230: return 221 # m-3m
+    return 1 # Default to P1
+
+def run_export(params):
+    import warnings
+
+    import gemmi
+
+    warnings.filterwarnings("ignore", category=UserWarning, module="mdx2")
+
+    from loguru import logger
+
+    logger.info("Loading HKL table from {}...", params.hkl)
+    hkl_table = loadobj(params.hkl, "hkl_table")
+    df = hkl_table.to_frame()
+    ndiv = hkl_table.ndiv
+
+    logger.info("Loading crystal geometry from {}...", params.geometry)
+    crystal = loadobj(params.geometry, "crystal")
+
+    # Scale fractional indices to integers
+    H = np.round(df["h"].values * ndiv[0]).astype(np.int32)
+    K = np.round(df["k"].values * ndiv[1]).astype(np.int32)
+    L = np.round(df["l"].values * ndiv[2]).astype(np.int32)
+
+    I = df["intensity"].values.astype(np.float32)
+    SIGI = df["intensity_error"].values.astype(np.float32)
+
+    # Remove reflections with non-finite intensity or sigma
+    mask = np.isfinite(I) & np.isfinite(SIGI) & (SIGI > 0)
+    n_total = len(H)
+    H, K, L, I, SIGI = H[mask], K[mask], L[mask], I[mask], SIGI[mask]
+    logger.info("Reflections: {} total, {} with valid I/SIGI", n_total, mask.sum())
+
+    uc = crystal.unit_cell  # [a, b, c, alpha, beta, gamma]
+    sg_symbol = crystal.space_group
+
+    # Scale a, b, c by ndiv to form the super-cell; angles are unchanged
+    super_uc = [uc[i] * ndiv[i] if i < 3 else uc[i] for i in range(6)]
+    if any(n != 1 for n in ndiv):
+        logger.info(
+            "ndiv={}: H,K,L scaled by ndiv; super-cell unit cell = {}",
+            list(ndiv),
+            [round(v, 4) for v in super_uc],
+        )
+
+    mtz = gemmi.Mtz(with_base=False)
+    mtz.title = params.title
+    mtz.cell = gemmi.UnitCell(*super_uc)
+
+    if any(n != 1 for n in ndiv):
+      sg = gemmi.find_spacegroup_by_name(sg_symbol)
+      laue_it = get_laue_it_number(sg.number)
+      logger.info(
+        "Supercell detected, changing space group to {}",gemmi.SpaceGroup(laue_it).hm
+      )
+      mtz.spacegroup = gemmi.SpaceGroup(laue_it)
+        
+    else:
+      mtz.spacegroup = gemmi.find_spacegroup_by_name(sg_symbol)
+    
+    ds = mtz.add_dataset("crystal")
+    ds.crystal_name = "crystal"
+    ds.project_name = "mdx2"
+    ds.wavelength = params.wavelength
+
+    mtz.add_column("H", "H").dataset_id = 0
+    mtz.add_column("K", "H").dataset_id = 0
+    mtz.add_column("L", "H").dataset_id = 0
+    mtz.add_column("IMEAN", "J")
+    mtz.add_column("SIGIMEAN", "Q")
+
+    # Add half-dataset columns if present
+    extra_cols = []
+    for j in range(2):
+        icol = f"group_{j}_intensity"
+        scol = f"group_{j}_intensity_error"
+        if icol in df.columns and scol in df.columns:
+            extra_cols.append((f"IMEAN_half{j + 1}", "J", df[icol].values.astype(np.float32)[mask]))
+            extra_cols.append((f"SIGIMEAN_half{j + 1}", "Q", df[scol].values.astype(np.float32)[mask]))
+
+    for label, col_type, _ in extra_cols:
+        mtz.add_column(label, col_type)
+
+    cols = [H.astype(float), K.astype(float), L.astype(float), I, SIGI]
+    cols += [arr for _, _, arr in extra_cols]
+    mtz.set_data(np.column_stack(cols))
+
+    mtz.write_to_file(params.output)
+    logger.info("Wrote {} reflections to {}", len(H), params.output)
+
+
+parse_arguments = make_argument_parser(Parameters, __doc__)
+run = with_parsing(parse_arguments)(with_logging()(run_export))
+
+if __name__ == "__main__":
+    run()
diff --git a/pyproject.toml b/pyproject.toml
index 481d075..bc65555 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -28,6 +28,7 @@ dependencies = [
     "simple-parsing",
     "loguru",
     "papermill",
+    "gemmi",
 ]
 # dxtbx needs to be installed with conda
 
@@ -54,6 +55,7 @@ Issues = "https://github.com/ando-lab/mdx2/issues"
 "mdx2.scale" = "mdx2.command_line.scale:run"
 "mdx2.reintegrate" = "mdx2.command_line.reintegrate:run"
 "mdx2.report" = "mdx2.command_line.report:run"
+"mdx2.export" = "mdx2.command_line.export:run"
 
 [tool.setuptools.dynamic]
 version = {file = "mdx2/VERSION"}
@@ -86,4 +88,4 @@ ignore = ["E741", "F541"]
 [tool.pytest.ini_options]
 addopts = [
     "--import-mode=importlib",
-]
\ No newline at end of file
+]