In this paper, we have developed an approximate formula for liquid saturation within a two phase condensing mixture that relates the saturation level to the slip velocity between the gas and liquid phases. In particular, we have explained why models in which the slip velocity is assumed to be zero exhibit a saturation that is several orders of magnitude smaller than in other models where slip velocity was allowed to vary. This is a discrepancy that has appeared in computed results reported in the fuel cell literature, but which has not yet received a satisfactory explanation. We demonstrate that the reason behind the large discrepancy is rooted in the type of model used to treat the slip velocity between phases. Copyright (C) 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.