The gas sensing behavior of praseodymium doped cerium oxide (PrxCe1 − xO2 or PCO) has been examined for 0–1000 ppm CO or H2 in a 10% O2 atmosphere at temperatures ranging from 250–350°C. Total conductivity as a function of temperature suggests that oxygen diffusion kinetics are slow below approximately 350°C. Devices with x = 0.05 and 0.10 show stable, n-type gas sensing response, while those with x = 0.20 exhibit significant drift in sensor output, presumably due to bulk oxygen migration. The response to CO is significantly stronger than that to H2 at 300°C, and at 350°C the response to H2 is nearly zero, resulting in a CO-selective gas sensing element. Suggestions for the source of selectivity in PCO are presented.