Experimental and theoretical studies were conducted to investigate the performance of a membrane-based energy recovery ventilator, which is an air-to-air heat exchanger with a water vapor permeable core. Tests were done on a commercially available membrane-based energy recovery ventilator, during which the supply and exhaust air states were recorded continuously over a long period of time. The test results show that as time passes the sensible effectiveness decreases very slightly from 56% to 55%, while the latent effectiveness increases appreciably from 28% to 39%. As a resultant of sensible and latent effectiveness, the enthalpy effectiveness provides an intermediate value between them and increases from 39% to 43%. The reason that the effectiveness changes with time is that the air relative humidity changes with time, which alters the moisture content at the membrane surface. Calculations were then implemented to predict the performance of the membrane-based energy recovery ventilator based on the model we developed previously and the calculation results were compared with the experimental data. The comparison suggests that the calculated effectiveness agrees well with the measured one, supporting the reasonability of the model. Calculations were also made to investigate the effects of various membrane parameters on latent and enthalpy effectiveness and latent-to-sensible heat ratio.