Thermal decomposition of guanidinium perchlorate has been studied by thermogravimetry, differential thermal analysis, mass spectrometry and X‐ray diffractometry. The title compound undergoes crystallographic phase transformation at 180°C before melting at 255°C. It decomposes exothermally into gaseous products in the temperature range 275°C–325°C. The mass spectral results suggest that the compound undergoes thermal decomposition into neutral particles which are then vapourized, ionized and oxidized. The fragment cyanamide is found to undergo trimerization to give melamine. Copyright © 1983 Verlag GmbH & Co. KGaA, Weinheim