The oxygen transport process in c-axis oriented YBa2Cu3O7-δ thin films was investigated in real time during deposition and postdeposition oxidation using in situ spectroscopic ellipsometry (SE). Two transport regimes dominated by oxygen out- and in-diffusion have been observed during deposition by ion beam sputtering at 700 °C. The effect of oxygen partial pressure on the extent of oxidation of as-deposited films has also been studied during the postdeposition cooling process. The thermodynamic stability of the grown films was examined by real time SE during the postannealing process. The relaxation time for oxygen diffusion has been found to decrease with higher oxygen partial pressures. The imaginary part of the pseudodielectric function <Є2> at an absorption peak was quantitatively correlated to the oxygen concentration. The pseudodielectric functions of oxygen deficient YBa2Cu3O6 films in the temperature range from 27 to 700 °C have been modeled using Lorentz oscillators. © 1999 American Institute of Physics.