Dense CeO2-doped with 10 mol% Pr (Ce0.9Pr0.1O2, PCO) films were fabricated as sensing electrodes of YSZ-based gas sensors by using radio-frequency magnetron sputtering method, and their sensing properties to toluene were evaluated in this study. The microstructure of the SEs was basically dense, but the rod-like microstructure was observed when the chamber pressure during sputtering was low. The toluene response of the sensors attached with the rod-like structured PCO films showed a higher increase rate with toluene concentration than those with the dense-structured PCO films in the higher toluene-concentration range. On the other hand, the toluene response of the sensors attached with the dense-structured PCO films was much larger than with the rod-like structured PCO films in the lower toluene-concentration range. The electrochemical impedance measurements indicated that the toluene response is comprised of electrochemical reactions between the triple phase boundaries (PCO/YSZ/gas) and the double phase boundaries (DPBs, PCO/gas). In addition, the increased density of the thin PCO film enhances the contribution of DPBs on toluene response, resulting in the increased toluene response in the lower toluene-concentration range.