Thallium(III) was precipitated with oxalic acid in the presence of 0.025M RbNO3 (or 0.0125M Rb2SO4) in 0.1M HNO3 (or 0.05M H2SO4). Chemical analysis of the solid obtained corresponds to the formula Rb[TlIII(C2O4)2]·4 H2O. Thermal studies (TG, DTG and DTA) indicated the dehydration and redox decomposition of the thallic salt to the thallous salt, and finally to a mixture of rubidium carbonate and oxides of thallium(I) and thallium(III). Infrared absorption spectra, microscopic observations and X-ray diffraction data were used to characterize the original salt, as well as the intermediates formed during the thermal decomposition of the salt. On the basis of these results, the salt may be represented as: Rb[TlIII(C2O4)2(H2O)2]·2 H2O.