During exposure of thin amorphous WO3 films to UV light, both the optical absorptivity and the electrical conductivity increase. Using optically excited surface plasmons, we were able to measure even subtle changes of the absorptivity of 10 nm thick films simultaneously with their electrical resistivity at various temperatures. Our measurements show that the temperature dependence and the dependence on the density-of-states of the electrical conductivity in WO3 can be explained by variable-range hopping of charge carriers between optically excited states near the Fermi level. Accordingly UV irradiation of tungsten oxide can be understood in terms of a photoinduced doping process which may have relevance for technical applications.