The process of spontaneous thermal dissociation of the semiconducting non-stoichiometric compounds Cu2-δX (X=S, Se) in vacuum has been investigated. Thermogravimetric, X-ray and electrochemical techniques reveal that the dissociation rates sharply decrease at δ < δcr(δcr=f(T)) and the chalcogen fluxes from the compounds into the vacuum become immeasurably small. The results of the experiments are discussed in terms of the formation of a phase at the surface with properties of a compound of constant composition. Possible reasons are the excess surface energy together with the high mobility of copper ions which results probably in the reconstruction of the subsurface layers.