Equations are derived from a generic, transient PEMFC contamination model to predict the effect of CO contaminant concentration in H-2 on both steady state performance losses and time constants. The resulting predictions allowed determination of the CO tolerance limit for the cases of a Pt and WC catalyst under specific operating conditions. An increase of the International Organization for Standardization CO tolerance limit of 0.2 ppm is possible because the CO concentration leading to a steady state performance loss of less than 1% is estimated at 0.2-0.9 ppm. An increase in CO tolerance limit is expected to reduce the analytical verification cost (quality control). The steady state performance loss is independent of catalyst loading thus avoiding a future standard change resulting from PEMFC cost reduction activities. (C) 2010 Elsevier Ltd. All rights reserved.