The thermal decompositions of thorium(IV) trimesinate and trimellitate, prepared under various conditions, have been studied. On heating benzenetricarboxylates of Th(IV) decompose in many stages. First, the hydrated complexes lose crystallization water in one, two or four stages, forming anhydrous comtlexes which decompose to ThO2 with the intermediate formation of ThOCO3. The dehydrations of the complexes are associated with strong endothermic effects, and the decompositions of the anhydrous complexes are accompanied by exothermic effects.