The effect of Co:Ni ratio on the oxidation performance of γ-γ' alloys based on the (Ni,Co)75Al5Ti5Cr15 system (at.%) was investigated. Seven model superalloys of Co concentration 0, 9, 19, 28, 38, 47 and 56 at.% underwent isothermal oxidation in air at 800°C using both box-furnace exposure (1000 hours) and thermogravimetric analysis (100 hours). Following 1000 hours oxidation at 800°C, alloys containing 28, 38 and 47 at.% Co exhibited a flatter, more compact external scale and Cr2O3 layer as well as reduced oxygen ingress compared with alloys of lower Co content. The alloy containing 47 at.% Co exhibited two distinct morphologies.