Does pore-size distribution need to be considered to accurately model vapor transport in membrane distillation (MD)? This paper addresses that question from a theoretical perspective. Although some previous work has discussed pore-size distribution in MD, there has yet to be a comprehensive, general analysis of its effects on MD and its various configurations. In this work, a numerical model is used to calculate the flux through all pore sizes to estimate the effect of pore-size distribution on MD flux. The modeling shows that the error in the calculated flux incurred by neglecting pore-size distribution is largest for a microfiltration process, where viscous flow dominates, somewhat smaller for vacuum MD, where Knudsen flow dominates, smaller still for direct-contact MD, where molecular diffusion usually dominates, and smallest for air-gap MD, where the air gap dominates the overall mass transfer resistance. Considering a membrane with a mean pore size of at least 100 nm and a geometric standard deviation of the pore size of 1.2, the error is: 9% for vacuum MD, 3.5% for direct-contact MD, and less than 1% for air-gap MD. (C) 2010 Elsevier B.V. All rights reserved.