Improved flow regime stability and lower pressure drop may be possible in two-phase microfluidic heat exchangers through the use of a hydrophobic membrane for phase separation. Past research on vapor-venting heat exchangers showed that membrane mechanical and hydrodynamic properties are crucial for heat exchanger design. However, previous characterizations of hydrophobic membranes were primarily carried out at room temperatures with air or nitrogen, as opposed to liquid water and steam at the elevated operating temperature of the heat exchangers. This work investigates laminated PTFE, unlaminated PTFE, and nylon membranes and quantifies the permeability of the membranes to air and steam. The pressure drop across the membrane as a function of fluid flow rate and temperature characterizes the membrane permeability. This work will facilitate more focused experimental work and predictive modeling on optimizing membrane properties and will help with the development of more effective vapor venting heat exchangers.