Abstract A knowledge of the threshold oxygen level in liquid sodium necessary for the formation of NaCrO2 in sodium-steel systems is useful in the operation of fast breeder reactors. There is considerable discrepancy in the data reported in the literature. In order to resolve this, the problem was approached from two sides. Direct measurement of oxygen potential in the Na(l)-Cr(s)-NaCrO2(s) phase field using the galvanic cell In, In2O3/YDT/Na, Cr, NaCrO2 yielded: Δ G o2 = −800847 + 147.85 T J/mol O2 (657–825 K). Knudsen cell-mass spectrometric measurements were carried out in the phase field NaCrO2(s)-Cr2O3(s)-Cr(s) to obtain the Gibbs energy of formation of NaCrO2 as: ΔG o f,T(NaCrO2) = −870773 + 193.171 T J/mol (825–1025 K). The threshold oxygen levels deduced from G o f,T (NaCrO2) data were an order of magnitude lower than the directly measured values. The difference between the two sets of data as well as differing experimental observations from operating liquid sodium systems are explained on the basis of the influence of dissolved carbon.