It is known that thiazol acid exists in two polymorphic modifications, anhydrous and 1.5 hydrate. Since new crystal forms, i. e., 0.5 hydrated crystals have been discovered, the behaviors of these polymorphs in methanol . water mixed solutions were studied by the measurements of water contents in crystals and by XRPD (X-ray powder diffraction), and the relationships between transformations of ploymorphs were elucidated. The transformation of thiazol acid crystals depended on the composition of methanol and water. Transformation of 1.5-hydrated crystals did not occur in 0 to 30Vol% methanol solutions, but they transformed to 0.5 hydrated crystals in 50 to 80Vol% methanol solutions, and to anhydrous crystals in 85 to 100Vol% methanol soultions. By addition of 1Wt% anhydrous crystals as seed crystals, 1.5 hydrated crystals transformed to anhydrous crystals even in 80Vol% methanol soultion. 0.5 hydrated crystals did not show transformation in 80 to 95Vol% methanol solutions, but transformed to anhydrous crystals in 100Vol% methanol solutions. The transformation of anhydrous crystals to 1.5 hydrated crystals was observed in a to 40Vol% methanol solutions, but not in 50 to 100Vol% methanol solutions.