The porosity, thickness, skeletal and bulk density of a wide range of gas diffusion layers were measured using a buoyancy method. In this method, thin porous samples were weighed both dry and submerged in a wetting fluid, allowing their solid volume to be determined by application of Archimedes principle. The results showed that GDL porosity decreased as the amount of hydrophobic polymer additive was increased. In general, the observed decrease in porosity was in agreement with the theoretical pore volume reduction calculated for a given PTFE loading. A simple mass-based analysis was performed to estimate the amount of PTFE in a given sample, which revealed that materials generally do not possess the PTFE loading they are reported to have, a fact that was confirmed by thermogravimetric analysis. When the measured porosity values were plotted against these improved estimates of PTFE loading, the agreement with the theoretically expected porosity trend was excellent. Comparisons were also made to gas pycnometry. Finally, measurements were made on samples cut from various locations on a given sheet. It was found that although thickness and areal mass varied between locations, the porosity remained relatively constant. (C) 2015 The Electrochemical Society. All rights reserved.