AG337 (Thymitaq) is an antitumor compound synthesized by Agouron Pharmaceuticals, Inc., using protein structure-based drug design. AG337 is a hydrochloride salt with a molecular formula of C14H12N4OS.2HCl. This compound was subjected to thermal analyses, Karl Fischer titrimetry, powder X-ray diffractometry, scanning electron microscopy, and FTIR. On the basis of the Karl Fischer and thermogravimetric analysis, it was conclude to be a dihydrate. The differential scanning calorimetric studies revealed, that on heating, AG337 dehydrates and form a metastable form with a melting point of 213 degrees C followed by crystallization into a stable form at 261 degrees C. This stable form was finally melted at 312 degrees C with decomposition. On the basis of the FTIR and HPLC studies, it was concluded that the final exothermic peak at 320 degrees C was due to sample decomposition. The powder X-ray diffraction studies confirmed the existence of these two polymorphs of AG337. Scanning electron microscopic studies revealed that the crystal habits of both the polymorphs were quite different. FTIR spectra of both the polymorphs showed pronounced difference in the range of 600-1800 cm-1.