The anhydrous solvent-free mechanochemical reaction of sulfathiazole, STZ, polymorphs I, III, and V with 10 carboxylic acids was monitored by powder X-ray diffraction (PXRD), attenuated total reflectance infrared (ATR-IR), and near-infrared (NIR) spectroscopy. A 1:1 cocrystal was observed with glutaric acid and the strongest acid, oxalic acid, gave a 1:1 salt. A principal components analysis of the glutaric acid NIR data showed that forms I and V proceeded to the cocrystal, but that form III transformed to form IV before cocrystal formation. The oxalic acid salt was formed via complete amorphization. The crystal structures of the cocrystal and the salt were determined. Sulfathiazole comilled with L-tartaric and citric acids gave coamorphous systems that were stable at 10% RH for up to 28 days. Comilling sulfathiazole with DL-malic acid gave mixtures of form V and the amorphous form.