In support of the selection of structural materials for heat exchangers in helium-cooled high temperature reactors, the oxidation behaviour of the Ni-base chromia-former alloy 230 was investigated at 850 A degrees C in diluted helium atmosphere with a low water vapour content. In such a media, the equivalent partial pressure of oxygen (imposed by the 850 degrees C in diluted helium atmosphere with a low water vapour content. In such a media, the equivalent partial pressure of oxygen (imposed by the P(H2O)/PH(2) ratio) is very low (P(O2)(eq) around 10(-16) Pa). The equivalent partial pressure of oxygen has no straight influence on the parabolic rate constant (k(p)); on the other hand, P(H2) and P(H2O) demonstrate a complex influence on k(p). Photoelectrochemistry analyses revealed that this oxide could simultaneously contain two types of cationic defects. Specific oxidation tests with D(2)O showed that the oxide scale also contains hydrogen. A mechanist model is proposed in order to describe the scale growth using both cationic defects. Those theoretical results show, at least qualitatively, how PH(2) and PH2O may concurrently influence the oxidation rate.