We report a new characterization method that detects liquid water breakthrough in porous materials by measuring changes in relative humidity and electrochemical impedance simultaneously. Unlike previous work based on capillary pressure, a four-chamber tool was developed to monitor the pressure and dew point temperature across porous transport layer materials. This approach can characterize the structure and water transport behavior of carbonaceous materials for polymer electrolyte fuel cells and electrolyzers. The changes in the dew point, impedance, and pressure provide a reliable and fast indicator to predict the liquid water breakthrough. © 2015 The Electrochemical Society. All rights reserved.