A modified electrochemical impedance spectroscopy (EIS) method has been developed to probe fundamental hydration properties in polymer electrolyte membrane fuel cells (PEMFCs) related to subzero cold-start phenomena. Subzero cold-starts are vital to widespread automotive PEMFC commercialization. The EIS method is specifically sensitive to the charge transfer and diffusional resistances of the membrane electrode assembly (MEA). The lower initial water content left perpendicular lambda = 2.7 vs 3.2 (mol H2O/SO3-)right perpendicular of the PEMFC negatively impacted the initial ohmic and charge transfer resistances. Isothermal water fill tests (WFTs) were performed at 10 mA/cm(2) to make discrete increases in water content within the MEA. The initial water content affected water uptake of the ionomer (lambda = 2.7 vs 3.2 was almost double) and the change in ohmic and charge transfer resistances. EIS performed at three stages-pre-WFT, hydrated, and freeze-out-show specific changes. In initial WFTs, the ohmic resistances decrease to similar values (similar to 7 x for lambda = 2.7, similar to 1.5 x for lambda = 3.2). Diffusional changes are negligible. In freeze-out WFTs, ice forms in the catalyst layer. An increase in the ohmic resistance and almost tripling of the diffusional resistance were observed for lambda = 2.7 and 3.2. Ice produced in the void volume and changes in the diffusional resistance were not significantly affected by the preconditioned initial water content (lambda = 2.7 vs 3.2). (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3525601] All rights reserved.