This article is an examination of the CO poisoning and cleaning (stripping) phenomenon that occur in a PEM fuel cell operating on an impure hydrogen stream such as reformed hydrocarbons or alcohols. A range of experimental results including cell polarization curves, measurements of spontaneous and transient oscillations of the anode potential and current pulsing behaviour are presented. Detailed examination of the pulsing process has shown that optimization of both the pulse width and pulse initiation potential will have an important impact on the overall fuel cell efficiency. To optimize these processes, the development of a mathematical model to understand and control the poisoning and cleaning processes is going to be important. In this paper, we have extended the model of Zhang et al. [J. Zhang, Investigation of CO tolerance in proton exchange membrane fuel cells, PhD thesis, Worcester Polytechnic Institute, June, 2004; J. Zhang, R. Datta, J. Electrochem. Soc. 149 (2002) A1423; J. Zhang, J.D. Fehribach, R. Datta, J. Electrochem. Soc. 151 (2004) A689] to include mass transfer effects. It is shown that this new model gives results that are in reasonable agreement with our experimental data. (C) 2007 Elsevier B.V. All rights reserved.