The effect of oxygen backfilling on WO3 electrochromic thin films has been studied. Previously it was shown that O2 backfilling reduced the corrosion rate in H2SO4. In this phase of the study it has been shown that O2 backfilling also affects the EC response. For example, the coloration speed and optical efficiency were decreased while bleaching speed and self-erasure rate were increased. The current injection during coloration was found to be controlled by the same mechanism as the films without backfilling (i.e., WO3 resistivity and Helmholtz double layer). However, the magnitude of the current passed was lowered due to increased film resistivity from backfilling. The optical absorption maximum remained unchanged while the thermal hopping energy increased with oxygen backfilling. These observations suggest that a decreased density of optically active absorption centers resulted in the lower optical efficiency.