Positron lifetime measurements have been applied to identify the type of defects and the structure of defects produced in the gamma-phase copper based alloys having non-stoichiometric composition. For the gamma-phase Cu-Al and Cu-Ga, vacancies are introduced in the specimens when the concentration of Al and Ga exceeds its stoichiometric composition. For the gamma-phase Cu-Ga, vacancies are also introduced in the specimens even when the concentration of Ga is lower than the stoichiometric composition. The result suggests the formation of non-stoichiometric vacancies at both Cu and Ga atom sites in the gamma-phase Cu-Ga. Anti-site atoms, however, play an important role to maintain non-stoichiometric composition in the gamma-phase Cu-Al and Cu-Ga.