An investigation of the practical viability of Pt on CeO2-containing supports as water gas shift catalysts is reported. This paper takes into account the anticipated duty cycles for residential fuel processors integrated to a PEM fuel cell. It was recently found that CeO2 based catalysts deactivate upon start-stop operation of fuel processors due to carbonate build-up and we suggested that ZrO2-CeO2 composites may be a better choice as the support. Aging of Pt-based ZrO2-CeO2 supported WGS catalysts however is observed under relatively mild operating conditions. We investigated the reasons for aging observed under steady-state operation, high temperature stress and start-stop cycling for a Pt/CeO2-ZrO2 water gas shift catalyst by activity testing, transmission electron microscopy (TEM), IR and chemisorption techniques. Substantial aging was observed under both steady-state operation at relatively low temperature (200-250 degrees C) and high temperature stress (up to 450 degrees C), whereas almost no reduction in activity was seen by simulating start-stop cycling, unlike with Pt/CeO2. The loss of activity could be correlated with Pt-dispersion measured by CO chemisorption and TEM. Very little formation of hydroxycarbonate and carbonates on the ceria part of the support was observed. (c) 2006 Elsevier B.V. All rights reserved.