A mathematical model was developed to describe the coupled heat and mass transfers in membrane processes. Equations for the heat and mass transfer resistances were derived and the coupling effects of the heat and mass transfer were analyzed. With taking the membrane separation process of moist air as an example, the effects of air temperature and water vapor concentration on the heat and moisture transfer process were investigated. The results show that neither the thermal resistance nor the moisture resistance are constant, they are affected by not only the membrane parameters but also the air state. As the temperature difference between the two airstreams separated by the membrane increases, both the thermal and moisture resistances decrease, causing an improved heat and mass transfer. As the average temperature of the two airstreams increases, the thermal resistance remains almost constant while the moisture resistance decreases significantly. Further, as the water vapor concentration difference between the two airstreams increases, both the thermal and moisture resistances increase. As the average water vapor concentration of the two airstreams increases, the thermal resistance remains unchanged while the moisture resistance decreases.