Formation of the Co/Si(111)7 x 7 interface has been investigated by methods of electron energy loss spectroscopy (EELS) and Anger-electron spectroscopy (AES). The sequence of surface and thin-film phases at d = 0-1, 2 and 3 ML, and Co-terminated bulk phases (CoSi2, CoSi, Co2Si) at d = 4, 6-11, and 13-30 ML has been established. It has been shown that the structure of phases at d = 0-1 ML changes during annealing, while that at d = 2 ML does not change. At d = 3 ML, a phase extending on thickness with layered structure, which transforms into CoSi2 at T > 450 degrees C, has bean detected. This phase type grows in a wide composition range up to Co thickness of 11 ML under the layer-by-layer Co deposition and annealing.