A straight, single channel membrane humidifier was constructed to measure temperature and moisture profiles along both the donor and receiver channels. A persulfonic Nation membrane was used as the water exchange medium. We report on results obtained with single-phase vapour-to-vapour, counter flow operation. First, the heat loss to the surroundings was quantified and found to affect the overall performance significantly. Second, the results from varying flow rates indicate that lower flow rates lead to higher outlet dew point values of the receiver stream which can be related to longer residence times. It was also found that moisture transfer is more strongly influenced by the flow rate through the receiver side than the donor side. Finally, five-point dew point profiles for both donor and receiver sides are reported for various temperature conditions. No stream wise variation in moisture flux was observed, and the average flux value increased from 3.3 x 10(-5) kg s(-1) m(-2) at 30 degrees C to 2.0 x 10(-4) kg s(-1) m(-2) at 70 degrees C under fully humidified donor-side inlet conditions. (C) 2007 Elsevier B.V. All rights reserved.