The process was carried out in a plate and frame module with a poly(vinylidene fluoride) membrane. Experiments were performed with the feed and permeate temperature kept respectively at 333 K(or 343 K) and 293 K. During membrane distillation (MD) of fluosilicic acid solutions, the permeate flux declined from 385 to 11.4 dm(3)/(m(2) d) with an increase of acid concentration in the feed from 2 to 35 wt %, at T-F,T-in = 343 K. The retention coefficient was close to 92%. A combination of membrane distillation with osmotic membrane distillation was also studied. In this case a saturated brine was used as the cold solution. Under these conditions the driving force (partial pressure difference) was higher than that for pure MD. MD experiments of a solution containing both fluosilicic and phosphoric acids, with the acid concentration of 6% H2SiF6 and 30% P2O5, respectively, were also performed. At such conditions the water vapor as well as volatile fluorine compounds are transported through a hydrophobic membrane from the feed solution to the cold distillate. The distillate was then a fluosilicic acid, practically free of phosphates. The investigations with a raw technical fluosilicic acid were also performed.