Abstract Thorium dioxide samples have been submitted to homogeneous four-beam laser pulses in an autoclave filled with different gases. Phase transitions were analysed through the produced thermal arrest stages, which were detected by a precise multichannel pyrometer. The spectral emissivity of the samples, measured on line during heating up to 4000 K, is also reported in the paper. Thoria exhibits a melting point at 3651 K, and a pre-melting transition at 3090 K. These temperatures are weakly influenced by chemical reduction of the sample. Whilst virtually stoichiometric thoria melts congruently, in the hypostoichiometric oxide, the liquids and solidus temperatures could be clearly observed at O/Th = 1.98. The analysis of the measured data and, in particular, of the heat capacity shows that the features of the pre-melting transition can be described by a Debye-Hückel oxygen defect model, where co-operative effects lead, in the oxygen sublattice, to a critical order-disorder transition, whose thermodynamic nature depends on the parameters of the defect mean-field coulomb interaction. A comparison of the high temperature phase diagrams of thoria and urania indicates close analogies in the pre-melting behaviour of these two compounds.