Heat and mass transfer mechanisms in a cross-flow parallel plate membrane-based enthalpy exchanger for heat and moisture recovery from exhaust air streams are investigated. The flow is assumed laminar and hydrodynamically fully developed, but developing in thermal and concentration boundaries. Contrary to the traditional methods to assume a uniform temperature (concentration) or a uniform heat flux (mass flux) boundary condition, in this study, the real boundary conditions on the exchanger surfaces are obtained by the numerical solution of the coupled equations that govern the transfer of momentum, thermal energy, and moisture in the two cross-flow air streams and through the membrane. The naturally formed heat and mass boundary conditions are then used to calculate the local and mean Nusselt and Sherwood numbers along the cross-flow passages, in the developing region and thereafter. A comparison was made with those results under uniform temperature (concentration) and uniform heat flux (mass flux) boundary conditions, for rectangular ducts of various aspect ratios. An experiment is done to verify the prediction of outlet moisture content. (c) 2006 Elsevier Ltd. All rights reserved.