Effects of the developing entrances on the fluid flow and conjugate heat and mass transfer in membrane-formed parallel-plates ducts used for liquid desiccant air dehumidification are investigated. The air and the liquid desiccant streams are separated by the semi-permeable membrane to prevent liquid droplets from crossing over. The two streams exchange heat and water vapor through the membrane in a cross-flow arrangement. In practical applications, due to the confinements in noises and pressure drops, the duct lengths are limited. The developing entrance lengths account for a large proportion of the duct lengths. Any assumption of either hydraulically fully developed, or thermally or/and concentrationally fully developed flow would underestimate performances seriously. In this study, a more accurate method is proposed. The equations governing the fluid flow and conjugate heat and mass transfer in the membrane ducts are established and solved directly considering the influences of the flow, and heat mass transfer developing entrances. The friction factors, Nusselt and Sherwood numbers along the duct lengths are obtained. The effects of the developing entrances on the fundamental data are investigated. Experimental work is conducted to validate the results. (C) 2013 Elsevier B.V. All rights reserved.