The effects of active (carbon dioxide and carbon monox{de) and neutral (helium, argon and nitrogen) atmospheres on the course of thermal decomposition of oxalates have been studied and compared. A mechanism of thermal decomposition has been proposed on the basis of the results obtained, the first stage of which consists in a heterolytic dissociation of the C~ bond, with the formation of carbon dioxide and the CO~-anion.In our earlier studies on the systematics of thermal decomposition mechanisms, it was shown that heterolytic cleavage is the first stage in the thermal decomposition of salts with bicentric anions containing a direct electron bond between the central atoms and a full coordination by oxide ligands (e.g. hyposulphates, $2062-, and hypophosphates, P20~-) [1, 2]: S2062-~SO3 + SO~-P20~ ~PO; +P033-Since carbon is an element of the previous period, oxalates are analogues of hyposulphates and hypophosphates and in the first stage they should decompose analogously: