The films with the extreme concentration of poly (diallyldimethylammonium chloride)/sodium dodecylsulfate complex spread to the surface of aqueous solutions of formic and acetic acids reduce the evaporation rate of liquids by 3–6% and demonstrate selective properties increasing water content in the vapor by 1–3 abs.%. The concentrations of solutions were up to 30 vol.%. The formation conditions of such films were determined by means of the Wilhelmy plate technique. Surface tension isotherms of aqueous solutions of acids were also obtained.