Concentration of NaCl solutions by direct-contact membrane distillation (DCMD) integrated with salt crystallization has been studied. The salt crystallization was carried out in a batch mode or continuously. The influence of process parameters (flow rates, temperatures, and concentrations) on permeate flux has been investigated. To evaluate the polarization phenomena, a simple model was used. This model showed good agreement with the experimental results. Two types of membrane distillation MD capillary modules, with the membranes arranged in a form of braided capillaries or helically wounded, were tested. The membrane wettability during long-term testing was investigated. A slow decline of the module efficiency was observed since the membranes were partially wetted during the process. The MD process integrated with continuous crystallization yielded an average NaCl production of 100 kg m(-2) d(-1).