The thermal behaviour of the binary systems HNH2SO3-NH4NH2SO3 (I), HNH2SO3-(NH4)2SO4 (II), HNH2SO3-NH4HSO4 (III), NH4NH2SO3-(NH4)2SO4 (IV), NH4NH2SO3-NH4HSO4 (V), (NH4)2SO4-NH4HSO4 (VI) was investigated with a Du Pont Thermal Analyzer supplied with a DSC module. The corresponding single compounds and freshly prepared (NH4)2S2O7 were examined under the same conditions in preliminary experiments. The ΔHf values of HNH2SO3, NH4NH2SO3, (NH4)2S2O7 and (NH4)2SO4 · NH4HSO4 determined in the absence of air were 11.9, 16.8, 11.7 and 11.7 kJ/mole. Due to the moisture content of the atmosphere, an repeated heating in air the DSC curves showed more endothermic peaks than under air-free conditions. Exothermic additive reactions took place for binary systems (I) and (III), resulting in the formation of (NH4SO3)2NH and (NH4)2S2O7, respectively, and for systems (II) and (VI), producing a new compound on melting.