Conductivity anomalies are observed in mixed ion-polaron conducting glasses in the system Li2O:WO3:P2O5. Deep minima in conductivity (reminiscent of the mixed alkali effect) occur when small amounts of Li2O are introduced into WO3-x:P2O5 glasses. By contrast, glasses melted in air are always deep blue in colour, except when large amounts of Li2O are incorporated. It is suggested that negatively charged polarons (effectively the d-electron located on a W5+ centre) interact strongly with mobile cations to form uncharged diffusing entities. This may be a general phenomenon in mixed ionic electronic conductors where mobilities of ions and electrons are comparable in magnitude.