The occurrence of an apparent solubility of oxygen in polycrystalline NbC, TiC, VC and ZrC is discussed. This is shown by experiments in which the oxygen consumed by the sample is directly measured as a function of temperature in an argon stream at 1.6 bar (rel) where the oxygen partial pressure was as low as 0.8 Pa. The parameter Gamma, as the ratio between the weight change of the sample and the number of moles of oxygen consumed, has been proposed to discriminate the nature of the processes related to the gas/solid interaction. The limit values of Gamma, theoretically expected to be 0 and 32 g mol(-1), mean that the gas/solid interaction changes from a surface reaction without any weight change to a complete retention of oxygen in the solid. Intermediate values indicate the formation of the oxide at a given stoichiometry as found experimentally as a function of thr oxygen partial pressure.