The vaporisation behaviour of (U,Pu)O2 mixed oxides (Pu/M=0.25, 0.50 and 0.75, with M=U+Pu) was studied by means of mass spectrometry. Hyperstoichiometric samples were heated in a Knudsen cell up to T=2300K. The evolution of the uranium and plutonium bearing gaseous species was studied as a function of time in order to evaluate the congruent vapour composition. Ionisation efficiency measurements were performed and the partial pressures of the gaseous species involved in the vaporisation process were determined. The vapour pressure has also been calculated using a thermochemical model for the (U+Pu+O) system. A quasi congruent composition with respect to the O/M ratio has been assumed, in agreement with the experiments. Nevertheless, the evaluation of all the experimental and calculated results shows that a total congruent composition exists for a single composition of the mixed oxide (MOX) samples with a Pu/M content slightly lower than 0.50. A good agreement is obtained between the calculated and experimental vapour pressure data, as well as the quasi congruent vaporisation compositions.