Membrane distillation was suggested for the regeneration of volatile inorganic acids (hydrochloric and nitric acids) from the metal pickling solutions using hydrophobic porous and nonporous asymmetric membranes. The investigations were performed with a flat sheet commercial microfiltration membrane based on tetrafluoroethylene/vinylidenefluoride copolymer MFF-2 and with a plasma modified commercial asymmetric gas separation membrane based on poly(vinyltrimethyl silane) (PVTMS). The influence of acid concentration (varying over a wide concentration range from 0.01 to 70 N) and the feed temperature (T-h = 313-333 K) on the transmembrane flux and selectivity was studied. It was shown that the concentrating of volatile inorganic acids in the feed using a porous MFF-2 membrane is effective up to 1.0 N concentration. Plasma-modified PVTMS membrane was used for the separation of feed solutions with higher acid content up to 7.0 N. It was established that the plasma treatment essentially increases the hydrophilic properties of PVTMS membrane surface.