Temperature profiles in hollow fiber (HF)-direcL contact membrane distillation (DCMD) are modeled analytically using the standard perturbation theory and the method of separation of variables. The theoretical results explain experimental observations reported in the literature. For HF-MD, new basic quantities F-w and S-q are proposed instead of conventional mass and heat flux definitions, respectively. Temperature vanes linearly in the longitudinal direction, and F-w and S-q are proportional to the membrane thickness to the power of -2/3. The stream speed of the hot teed primarily controls mass transfer, and the lumen flow rate significantly influences the heat transfer. An analytical expression of the theoretical maximum membrane length is derived, which decreases with transmembrane temperature difference and increases with feed and permeate streams. The best performance of HF-DCMD operation in terms of mass and heal transfer can be reached by using a short HF membrane with fast stream speeds. For membranes of similar pore size and thickness. MD membranes of high porosity will give better performance by providing sparser pore structures and consequently lower spatial fraction for heat transfer. (C) 2014 Elsevier B.V. All rights reserved