Membrane distillation (MD) was applied for the concentration of solutions containing hydrochloric acid and potassium chloride. The studies of the concentration and separation of HCl through the hydrophobic membrane were performed. In the investigations plate and frame or capillary modules, equipped with membranes from polytetrafluoroethylene and polypropylene were applied, respectively. The feed temperature amounted to 333 K or 343 K and permeate 293 K at the inlet of the module. Under the MD operation conditions the transfer of water vapour and hydrogen chloride from potassium chloride and hydrochloric acid solutions through the hydrophobic membranes took place, whereas the potassium chloride as a nonvolatile component underwent concentration in the feed. The influence of acid concentration and salt presence in the feed on the HCl molar flux through the membrane was systematically studied. The increase of the temperature and salt concentration in the feed caused higher partial pressure volatile of HCl and the resultant HCl Flux through a membrane was higher.During MD for the initial concentration in the feed equal 50 g KCl/dm3 and 40g HCl/dm3, at the feed temperature 343K, the permeate flux decreased from 353 dm3/m2d to 289 dm3/m2d, whereas the HCl flux increased to 6 mol/m2d for the capillary module. At higher KCl concentration in the feed, amounting to 100g KCl/dm3 and under the same operation conditions, the permeate flux decreased to 285 dm3/m2d, but the molar flux HCl increased to 18 mol/m2d. The results were compared with the data obtained for the plate and frame module.