The latent effectiveness and the latent number of transfer units (NTUs) for mass transfer in membrane humidity exchangers were applied to proton exchange membrane fuel cell (PEMFC) membrane humidifiers. We report on two limitations that cause deviations in the theoretical outlet conditions reported previously: (1) using a constant enthalpy of vaporization derived from the reference temperature in the Clausius-Clapeyron equation; and (2) simplifying the relationship between relative humidity and absolute humidity as linear. These limitations are alleviated by using an effective mass transfer coefficient U-eff. The constitutive equations are solved iteratively to find the flux of water through the membrane. The new procedure was applied to three types of membrane and compared to the curves of epsilon(L) and NTUL found using Zhang and Niu's method, which is normally applied to energy recovery ventilators (ERVs). (C) 2008 Elsevier Ltd. All rights reserved.