Selective transformation pathways between crystalline forms of an organic material established from powder X-ray diffraction analysis.

The methanol solvate crystalline phase (M) of benzene-1,2,4,5-tetracarboxylic acid transforms selectively to a hydrate phase (H) or a non-solvate phase (N) depending on the presence of atmospheric water vapour; knowledge of the crystal structures of M and N, determined here using single-crystal and powder X-ray diffraction, respectively, yields insights into mechanistic aspects of the solid-state transformations.