In a series of epitaxial La0.8Sr0.2MnO3- films 12–75 nm thick on NdGaO3 substrates, the variation in the oxygen content during heat treatment in air at temperatures of 873–1073 K was quantitatively studied. Experimental and calculated time dependences of the change in the oxygen content in films with taking into account the effect of inhomogeneous diffusion of oxygen over the thickness of the films, allowing to make the optimal choice of temperature and duration of the heat treatment procedure. In films 25–75 nm thick, the effect of “stabilization” of the oxygen content is observed, which indicates the presence of an energy barrier that prevents the incorporation of an excess oxygen atom into the surface layer of the film under optimal conditions of heat treatment.