The oxygen in-diffusion processes in c-textured epitaxial thin films were monitored by in situ electrical resistivity measurements. The chemical diffusion coefficients were found to be sensitive to the oxygen content and varied greatly on crossing the phase transition point. The varying diffusivity could be divided into three ranges where different diffusion mechanisms work. Diffusivity was concentration independent in orthorhombic phase limits (interstitial mechanism), decreased as oxygen deficiency increased up to the point of the O-T phase transition, and then increased with increasing oxygen deficiency in the tetragonal phase (vacancy mechanism). The active energies of oxygen diffusion were obtained to be about and in tetragonal and orthorhombic phases respectively; these values were close to those of other authors. The magnitude of chemical diffusivity variation was within one order at the same temperature.