The studies on water vapour and HCl transfer through the hydrophobic membrane during membrane distillation (MD) have been performed. The flat-sheet membranes made from polyretrafluoroethylene (PTFE) or poly(vinylidene fluoride) (PVDF) and polypropylene (PP) capillary membrane were used in the experiments. The performance of the applied membranes has been compared. It was found that the course of the concentration of hydrochloric solution was different as a consequence of the various membrane morphologies. However, for all membranes used, the final level of HCl concentration in the feed amounting 253 g HCl/dm(3) (T-F = 343 K) was achieved. It has been shown that during the concentration of HCl solutions by MD, the HCl content in the permeate may be higher than in the feed. A beneficial effect of salt in the feed on the separation of HCl from the solution has been demonstrated. Analysis of mass transport across the membranes was performed, taking into account a multicomponent diffusion through the stagnant air layer existing inside the pores of the membranes. The agreement between the results theoretically predicted and the experimental data was satisfactory.