In the selection of corrosion protection materials their water vapour permeability, particularly in the area of higher operation temperatures is important The permeation coefficient is a good method of defining the permeability. A method of determining the permeation coefficient has been defined and the permeation coefficient in the temperature range 50-90 degrees C has been determined for several rubber sheetings, reaction resin coatings as well as plastics which are used in the corrosion protection of chemical equipment. In case of the plastics as building materials the permeation coefficient is a very important factor since these materials are applied in many layers and the largest reduction,ir pressure of the penetrating medium at the active protection layer is needed, to offer the supporting layer an optimum protection. For this application the thermoplastic materials PP, PE or FEP are most suitable.