Wilhelmy plate technique was used to determine surface tension isotherms of aqueous solutions of formic and acetic acids with concentration up to 30 vol% and with a film of solution of polyelectrolyte/surfactant complexes spread to the surface. Complexes of sodium polystyrene sulfonate/dodecyl trimethyl ammonium bromide and poly (diallyldimethylammonium chloride)/ sodium dodecylsulfate were used. The formation conditions of films with the extreme concentration of the complexes were determined. The films reduce the evaporation rate of acids solutions by 3-6% and demonstrate selective properties increasing the water content in the vapor by 1-5 abs%. Evaporation tests carried out by means of helium blowing over surface of the liquid with spread film.