The treatment of effluents from the regeneration of cation exchanger by the membrane distillation (MD) process has been investigated. The composition of these effluents is complex due to the content of residual acid (1-50 g HCl/dm(3)) and different amounts of various salts (ranging in concentration from 10 to 40 g/dm(3)) including hardly soluble salts. Therefore, long-term investigations were carried out to evaluate the performance of the MD process and a fouling potential of treated effluents. The MD module exhibited a stable performance in terms of membrane permeability during 300 h of a continuous treatment of these effluents. However, the distillate composition was affected by a feed concentration. A low HCl flux was observed when the feed contained less than 50 g HCl/dml(3), which resulted in the concentration of acid in the distillate below 0.5 g HCl/dm(3). A significant enhancement of the HCl flux (above 500 mol/m(2)d) was achieved by increasing the acid concentration in the feed to a level of 150 g HCl/dm(3). Therefore, 3-4 fold concentrations were sufficient to achieve a high HCl flux for effluents containing 4246 g HCl/dm(3). A condition of high HCl concentration in the feed implies that the recovery of HCl from the effluent with a low acid concentration (e.g. 1 g HCl/dm(3)) is a difficult task. Moreover, it was found that the SO42-ion concentrations in the feed was a factor limiting the degree of effluent concentration. The formation of salt deposit was observed after exceeding the concentration of 0.6 g SO42-/dm(3), which caused a decrease of the MD module efficiency.