Epitaxial La0.5Sr0.5CoO3 and La0.7Sr0.3MnO3 thin films have been grown on (100) LaAlO3 substrates by the pulsed laser deposition method. After deposition the films were in situ annealed at 2 X 10(-6)-10 torr of oxygen atmosphere. Electrical resistivity measurements reveal that the La0.5Sr0.5CoO3 films are highly sensitive to the thermal treatments. With the oxygen pressure being reduced, a metallic-semiconducting transition is induced. We demonstrate that the oxygen content in the films may not be the only factor that controls the transition observed. In contrast, the La0.7Sr0.3MnO3 films are insensitive to the annealing and keep a high electrical conductivity even after being treated at 2 X 10(-6) torr of ambient oxygen. The different thermal stability observed for the two kinds of films is discussed in terms of their structural characteristics.