Perfluorinated polyether (PFPE) films spread spontaneously on concentrated sulfuric acid to form insoluble thin films that act as diffusion barriers to water vapor condensation. At acid concentrations between 89 and 95 wt %, the average Film resistivity for water vapor is 8.3 +/- 1.0 s/cm per mu m thickness of PFPE. This resistivity corresponds to a diffusion coefficient fbr water in PFPE of approximate to 6 x 10(-6) cm(2)/s. Thus, a several micrometer thick PFPE film resists water condensation with an efficiency greater than the evaporation resistance of many long chain alcohol monolayers on water. Below approximate to 90 wt % sulfuric acid, thin indentations appear in the PFPE Film and the resistivity drops to approximate to 4 (s/cm)/mu m near 84 wt % acid.