Oxidation tests have been undertaken in air on two experimental fourth-generation Nibased superalloys containing, respectively, 3 and 5 wt.% Ru. A Ru-free, third generation alloy (CMSX-10K) was also included in the test programme which used temperatures covering the range 750-1100°C. Where possible, comparisons were also made with literature data on the secondgeneration CMSX-4 alloy and with IN738. After an initial period of transient oxidation, the subsequent parabolic rate constants for the 3%-Ru alloy were similar to those for CMSX-4 at all temperatures above 750°C. At 1000 and 1100°C these were consistent with the formation and growth of an α-alumina layer. At 950°C and lower, protective oxidation was obtained through the formation of an Al-rich spinel rather than alumina but, again, no deleterious influence of Ru additions could be found, at least in the temperature range of 800-950°C. Anomalous behaviour was found, however, at 750°C in that the Ru-bearing alloys oxidised non-protectively through the formation of sub-surface pits in which the γ΄ phase within the alloy was oxidised in situ.