[en] Highlights: • Detailed oxidation study of W-free low density Co base superalloy is presented. • Oxidation is primarily controlled by outward diffusion of Co and Ni. • S/TEM combined with EDS is applied to understand the oxide phase evolution. • A mechanism has been derived from these studies. -- Abstract: The paper reports mechanism of high temperature oxidation of the new γ’-strengthened Co-30Ni-10Al-5Mo-2Ta (at%) alloy. The plots of mass gain vs. time show a parabolic growth behaviour at a temperature range of 650–800 °C. The cross-sections of the oxidised samples reveal formation of a complex multilayer of oxides. The top layer comprises of Co₃O₄ followed by a solid solution of (Co_1-x,Ni_x)O (x varying from 0-0.2). A middle layer evolves with time comprising hexagonal molybdate Co₂Mo₃O₈, metastable TaO and amorphous Al₂O₃. The γ’ dissolve near the interface exposing the γ solid solution with growth of amorphous Al₂O₃ along the grain boundary.