Cycling of relative humidity (RH) levels in polymer electrolyte membrane (PEM) fuel cells reactant streams have been reported to decay fuel cell performance. This study focuses on accelerated durability testing to examine different modes of membrane failure via relative humidity cycling. A single fuel cell with an active area of 42.25 cm(2) was tested. A Hydrogenics G50 test station was used to establish baseline cell with 840 h of degradation under high humidity idle conditions at a constant current density of 10 mA cm(-2). The membrane electrode assembly (MEA) contained a Gore (TM) 57 catalyst coated membrane (CCM) and 35 BC SGL gas diffusion layers (GDLs). During the test, in situ diagnostic methods, including polarization curves and linear sweep voltammetry (LSV) were employed. Also, ex situ tests such as ion chromatography, infrared imaging, and scanning electron microscopy were used to identify degradation mechanisms. For RH cycling cell, H-2-air inlet gases were alternated under dry and 100% humidified conditions every 10 and 40 min, respectively. Under idle conditions, operated at very low current density, a low chemical degradation rate and minimal electrical load stress were anticipated. However, the membrane was expected to degrade due to additional stress from the membrane swelling/contraction cycle controlled by RH. The degradation rate for steady state conditions (0.18 mV h(-1)) was found to be lower than under RH cycling conditions (0.24 mV h(-1)). Change in RH led to an overall PEM fuel cell degradation due to the increase in hydrogen crossover current and fluoride ion release concentration. This study advanced the development of 'diagnostics' for PEM fuel cells in that failure modes have been correlated with in situ performance observations. (C) 2011 Elsevier Ltd. All rights reserved.