Catalyst layers (CLs) in proton-exchange-membrane fuel cells (PEMFCs) facilitate electrochemical reactions and therefore play a critical role in cell performance. Absorption and desorption of water into both the CL ionomer and the CL pore structure are integral aspects of PEMFC water management and performance. In this work, the water uptake from both the vapor and liquid phases is examined experimentally. Specifically, the dynamic water-uptake behavior of the CL ionomer is investigated as a function of relative humidity, temperature, Pt-loading and pretreatment. The water content of the ionomer in the CL, even after pretreatment, is found to be significantly lower than that for the bulk ionomer membrane, yet with similar sorption time constants. Thus, there is probably substantially slower transport into the ionomer which is likely due to its interfacial character. From the liquid phase, measured capillary pressure - saturation relationships show that the CL has an appreciably hydrophilicity that is strongly dependent on the existence of cracks. These findings are critical to the understanding and optimization of water management and transport phenomena within PEMFCs. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.031209jes] All rights reserved.