The decomposition mechanism of bimetallic particles encapsulated in the Pt-Co/NaY zeolite system is discussed. The large decrease in hydrogen uptake observed in TPR measurements taken before and after mild oxidation is interpreted in the light of the decomposition of bimetallic particles. During mild oxidation, Co2+ ions are first enriched on the surface of the bimetallic particles, then they become detached and migrate to the hexagonal or sodalite cages leaving Pt2+-covered Pt-0 kernels in the supercage. No agglomeration or migration of these kernels to the external surface was observed.