An analysis of the resistivity and thermoelectric power of heavily doped Nb2O5 [over-all composition (Nb1−xWx)2O5 where x varies from 0.0025 to 0.15] shows that mixed valence semiconduction has been observed. The major factor determining the value of the thermoelectric power at high temperatures appears to be an entropy of mixing term. In samples with x>0.10, it is concluded that the electrons can be trapped on tungsten ions as well as niobium ions. The low-temperature resistivity data indicate that the conduction mechanism is not simply described at temperatures where the tungsten impurity ions are incompletely ionized.