It is shown that the variation of a(H2O) in the gaseous phase has little effect on the electric-field-induced drawing-in of WO3 into the inner surface of the CaWO4 ceramic. This suggests that WO3 sublimation and transfer contribute insignificantly to the phenomenon under study. The activity of oxygen gas was shown to strongly influence WO3 spreading in an electric field: the spreading rate increased with a(O2). The electrosurface drift mobility of WO3 depended on a(O2) as U-WO3 = Ka(O2)(1/4) similarly to the variation of the concentration of holes with a(O2). Possible explanations have been proposed.