- Abstract We report on the time development of EPR signals ( g =2.0026±0.0002) from C 60 films with various crystalline structure under air/light exposure. The time development consists of two clearly distinguished regions of fast and slow growth. Improvement of the film structure, and in particular the increase in grain size, leads to a deceleration of the “fast” growth. The results are explained assuming that EPR signal growth is controlled by oxygen diffusion, along grain boundaries and into grains, during the “fast ” and “slow” periods, respectively. Fast decrease of the EPR signal as a result of in situ pumping strongly supports this model and indicates a correlation between crystalline structure and oxygen diffusion in C 60 films. Such correlation is considered as one of the possible mechanisms which govern the semiconducting properties of the material.