The first aim of this work is to present a method in order to characterize a direct contact membrane distillation (DCMD) module, namely a method to determine the value of the parameters included in a commonly accepted transport model. The model includes membrane and module design characteristic parameters which have the same values regardless of feed used. The characterization method proposed here, includes DCMD measurements (both, water flux and evaporation efficiency measurements) using pure water as feed, and air-liquid displacement measurements. The latter are used to extract previous structural information about the membrane used. The method is applied to characterize two DCMD modules with different designs. The first module has open flow channels while the second one includes a spacer within the flow channels. The transport model together along with the obtained characteristic parameter values adequately predicts the water flux measured with both modules when different operating conditions were imposed and aqueous solutions of different solutes were used as feed. The results given here are for NaCl and sucrose feed solutions. They show that the membrane distillation performance is highly dependent on the design of membrane module, the specific separation being performed and the operating conditions (besides membrane characteristics). The second aim of this work is to study the water flux enhancement induced by the improvement of the fluid-dynamic conditions within the module with spacer in relation to the module with open channels. The dependence of flux enhancement on operating conditions (average temperature, temperature difference between feed and permeate solutions, type of solute, and feed concentration) is analysed. In the range of operating variables studied, the systems with high vapour pressure polarization have water fluxes especially sensitive to the fluid-dynamic changes induced by the spacer. (c) 2005 Elsevier B.V. All rights reserved.