Measurement techniques for the in situ investigation of two-phase transients in polymer-electrolyte membrane fuel cells (PEMFCs) are presented. Fundamental concepts from system theory, voltammetry, and electrical impedance spectroscopy are extended. This results in the introduction of potential-step voltammetry and nonlinear low-frequency impedance as tools for the investigation of the two-phase flow in the gas diffusion layer. Potential-step voltammetry and nonlinear low-frequency impedance are lean and quick alternatives to neutron radiography and provide valuable information on the change of the liquid saturation with time. The measurement results prove that the continuity assumption of current two-phase models is valid for the Toray TGP-H paper. The data provided is useful for model validation and two-phase parameter extraction beyond model comparison to averaged saturation levels. (c) 2008 The Electrochemical Society.