Tetragonal and rnonoclinic polymorphs of sulfathiazole-pyridine solvate ((C9H9N3O2S2C5H5N)-C-.) were crystallized from propanol-pyridine solutions. The two polymorphs are unstable under ambient conditions but can be stored for a long time under pyridine vapor. TG and DSC of freshly prepared samples prove the decomposition of the solvates with pyridine vapor and sulfathiazole 1 as products. Thermodynamic estimate shows that the decomposition can be considered just a sum of two processes: (1) high-energy sublimation of pyridine and (2) low-energy structural transformation of the remaining sulfathiazole. (C) 2004 Elsevier B.V. All rights reserved.