Models were derived for the scavenging effect of product liquid water on airborne proton exchange membrane fuel cell (PEMFC) contaminants. A time scale analysis of contaminant mass transfer processes, product water accumulation in the gas diffusion electrode, and dissociation reactions indicated that the contaminant saturates the product liquid water simplifying model derivation. The baseline model only accounts for contaminant solubility. A multi-scale extension to this model was derived for the presence of contaminant dissociation reactions within the product liquid water using SO2 as a model contaminant. The extended model demonstrates the large impact of dissociation reactions at low SO2 concentrations. For both models, explicit expressions for the average gas phase contaminant concentration within the fuel cell were also derived and can be used as a surrogate for the effective contaminant concentration to correlate the fuel cell performance loss and facilitate the definition of tolerance limits and filtering equipment. The model was validated using a non-operating PEMFC. The water was transferred from the anode to the cathode by thermo-osmosis. Model contaminants, methanol and SO2, were injected with an inert carrier gas to avoid reactions. The model proved to be acceptable with parameters approximately equal to published values. (C) The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4:0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: [email protected].