The dependence of the ionic current density on the mean field in oxides grown on tantalum in electrolyte was studied using ellipsometry to estimate the thickness. The field in the outer layer which is believed to grow due to metal ion motion is higher because of phosphate incorporation. The field in this layer was estimated by assuming that the field in the inner undoped layer was the same as the mean field at the same current density and temperature in films made in dilute sulfuric acid, which are less affected by electrolyte incorporation. The results indicate that the effect of phosphate on the ionic conductivity may be due to its reducing the permittivity and hence the effective field. It is pointed out that electrolyte incorporation gives two effects which contribute to the nonlinearity of log vs. plots. Thus, since the metal ion transport number increases with so does the proportion of the film which contains phosphate and requires a higher field. Also the concentration of phosphate increases with . The incorporation leads to history effects which are not removed by subsequent formation, since the profile of phosphate records the sequence of current densities used. The time dependence of the field on changing the electrolyte is consistent with the field locally determined by the local film material.