The high temperature behaviour of thallic oxide obtained from different source materials was studied thermogravimetrically. Initially the normal equilibrium reactions viz. (a) the disso­ciation of the solid Tl2O3 to Tl2O or a Tl2O—Tl2O3 solid solution and (b) the volatilization of the solid decomposition product, occur almost simultaneously. It has been observed that depend­ing upon the experimental conditions, viz. the heating rate, the material of the crucible and the state of dispersion of sample, an additional reaction takes place. This is due to the vaporization of the still unreacted but molten Tl2O3. These reactions though not clearly distinguishable in the direct thermogravimetric curve, are unambiguously resolved in differential thermogravimetry (D.T.G) plots and in confirmatory DTA runs. The effects of the various experimental parameters on the TG and DTG curves are discussed.