Electroless deposition of Co(W,B) films on Cu surface is a novel approach for a selective capping process without Pd activation. Study of the oxidation resistance of thin Co(W,B) protective layers is required since it might be exposed to oxygen between process steps or during the formation of an upper layer of low-K inter-level dielectric which is deposited from an oxygen containing plasma. Oxidation studies were performed at different temperatures in room ambient and in oxygen plasma environment as a function of the capping layer composition, film thickness and plasma composition. While various oxides were detected on Co(W,B)/Cu samples after anneal in air at elevated temperature there was no oxide detected on samples treated with oxygen plasma at room temperature. It was assumed that the oxidation of either Co alloy or the Cu under-layer in plasma is accompanied with the removal of oxidized species from the surface, probably by the oxygen ions, probably by sputtering. The critical factor in plasma oxidation seems to be the substrate temperature. Oxidation in air showed that introducing boron onto the Co alloy increased the oxidation resistance of the electroless capping layer on Cu.