The sensitivity of the electrical conductivity of platinized semiconducting oxides at room temperature to the surface state of supported Pt is demonstrated. Materials that exhibited only Pt-0 species, despite catalyzing CO oxidation at room temperature, showed no CO sensitivity of the electrical conductivity at room temperature. Moisture in air caused oxidation of finely dispersed Pt-0 to Pt-II. The resultant materials did not catalyze CO oxidation at room temperature but did show a room-temperature response of electrical conductivity to CO. These materials did not have a large effect of water vapor on the electrical conductivity; however materials that had been further oxidized, to develop Pt(IV) states on the surface, not only showed a room-temperature response of electrical conductivity to CO, but also showed a large effect of water vapor on the conductivity. We propose the following: surface Pt(II) and Pt(IV) act as band-gap trap states for electrons from the support, significantly decreasing the measured conductivity; the room-temperature CO response was due to CO adsorption onto Pt(II) surface sites resulting in electron transfer to the supporting oxide, measured as a conductivity increase of the support; and the effect of water vapor was primarily due to adsorption onto Pt(IV) surface sites. Study of BaSnO3 and BaFeO3 as the support showed that the effect could only be observed in the case where Pt states were found in the oxide band gap and the oxide conductivity was dominated by electron exchange with these states.