Thin layer chromatography followed by reflectance densitometry has been used to evaluate the stability of chlorpropamide‐urea during the fusion process. Urea was found to decompose to biuret and chlorpropamide to p‐chlorobenzenesulphonamide: several other unidentified decomposition products were detected. The energy for decomposition of chlorpropamide was 57·1 kJmol−1 for melts containing 15 and 30% chlorpropamide. Decomposition followed apparent first order kinetics. 1979 Royal Pharmaceutical Society of Great Britain