The electrical conductivity of 45xKO(1/2)-45(1- x)WO3-25NbO(5/2)-30PO(5/2) (x = 0.5, 0.6, and 0.7) glasses was investigated. Heat treatment of the sample under a CO atmosphere at temperatures below the glass-transition temperature introduced electrons into the glass as carriers, following which W6+ was reduced to W5+. The electrical conductivity increased by four orders of magnitude for the x = 0.5 glass and by two orders of magnitude for the x = 0.6 glass, whereas it only doubled for the x = 0.7 glass. The results suggested that the increase in the electrical conductivity of the glasses for which x = 0.5 and 0.6 was caused by hopping of small polarons, which were generated by nearby W5+, in an adiabatic process. However, the adiabatic small polaron theory could not be applied to the x = 0.7 glass, which we attributed to the change in the K+-ion conductivity in the case of the x = 0.7 glass, where the K+ ions contributed substantially to the electrical conductivity even before the heat treatment under a CO atmosphere. (C) 2013 Elsevier B.V. All rights reserved.