Thin fluorocarbon polymer films are prepared on PE-foils in low-pressure electron cyclotron resonance plasmas using ethylene (C2H4) and trifluoromethane (CHF3) as monomers. The thin fluorinated hydrocarbon layers strongly reduces the permeability of polyethylene to alkanes. For example, the permeation of toluene was decreased by a factor of about 100 by a single, thin fluorocarbon layer. A further reduction of the permeation down to a factor of 1600 can be obtained by a multilayer coating. X-ray photoelectron spectroscopy and Fourier transform IR spectroscopy are used to characterize the plasma polymerized films. It is shown that the addition of CHF3 to a C2H4 plasma leads to an increase of CF3-, CF2-, and CF- groups and to a decrease of CH3- and CH2- groups in the film. The chemical composition of the polymer layers and their toluene permeabilities are discussed.