The surfaces of vacuum deposited thin films change very easily under the influence of atmospheric environment. In dependence of the chemical reactivity of the metal oxide, sulphide, nitride etc. overlayers will be formed more or less rapidly and water will condensate which, however, by chemisorption changes the chemical composition of the metal/air interface too. The analysis of small changes of the interface, i.e. at only a few atomic lay- ers of impurities, can not be possible with usual bulk methods. The parameters of surface electromagnetic waves (surface plasmons, SPO), which propagate along the metal/insulator interface are, however, extremely sensitive to the physico-chemical composition of the metal layer.