Full crystal structural characterization of three crystal polymorphs of 2-iodo-4-nitroaniline was carried out: the triclinic, orthorhombic, and a new monoclinic form. Powder X-ray diffraction, differential scanning calorimetry, and IR data on the three of these are reported. Solvent-mediated transformations were obsd. on the basis of changes in crystal morphol. and data from an in situ laser probe. Transformation to the monoclinic form was obsd. in all cases.