The density of membranes in polymer electrolyte membrane fuel cells (PEMFC) is an important parameter that has not been widely measured. In this paper, water uptake, hydration enthalpy and density were measured for a series of equilibrated Nafion and 3M ionomer membranes with different acid groups and equivalent weights. A systematic method is developed for measuring the density of hydrated membranes and the statistics associated with the measurement are assessed. Partial molar volumes of water in different membranes are calculated from the density data and used to analyze water/polymer interaction during water uptake process. Several aspects of the thermodynamics of these systems are reconciled and some mechanistic aspects of water uptake are elucidated. (C) 2017 Published by Elsevier Ltd.