We report on the metal-insulator transition ofLa0.5Sr0.5CoO3thin films deposited by pulsed laser ablation onLaAlO3substrates. The films were cooled in oxygen partial pressures between 760 and10−5Torr and electrical resistivity of the films was measured as a function of cooling oxygen pressure.La0.5Sr0.5CoO3films changed from metallic to insulating behavior depending on their oxygen content. A defect model has been proposed to explain this transition and the change in conductivity is related to the change in the oxidation state of the cobalt ions. The model explains the relationship between oxygen partial pressure and electrical conductivity inLa0.5Sr0.5CoO3, which describes the experimental dependence reasonably well. Positron annihilation studies were also done on the same set of samples and the S parameter was seen to increase by 8&percent; from a fully oxygenated sample to a sample cooled in10−5Torr. ©1997 American Institute of Physics.