The thermodynamic isotope effect H/D/T for proton conducting oxides in equilibrium with gas phase was theoretically considered. The difference in concentrations n of hydrogen, deuterium and tritium in the same samples exposed to gas phases containing the vapors of only one of these elements was analyzed. The oxides of the A(II)B(1-x)(IV)R(x)(III)O(3-y) type were considered in the frame of some known defect formation models. It was shown that for a characteristic change in the proton-associated vibration frequencies with isotopic substitution of protons, the concentrations n(H), n(D) and n(T), under equivalent external conditions, might be appreciably different. The effect essentially depends on the external conditions and on the ratio of nil to dopant concentration x: it is maximal at low n(H), and disappears when n(H)/x --> 1. A possibility of direct experimental observation of the thermodynamic isotope effect in perovskite oxides is briefly discussed. (C) 2003 Elsevier B.V. All rights reserved.