Thin films of Pb0.8Sn0.2Te of different thicknesses have been vacuum deposited by flash evaporation onto glass substrates held at room temperature. Electrical resistance measurements as a function of film thickness have been carried out on these films in the temperature range 300-500 K. It was found that during the initial heating the resistance varies irreversibly showing an anomalous behaviour. This behaviour has been explained by considering the chemisorption of oxygen on the film surface. During the cooling cycle, thin film conductivity exhibits the expected reciprocal temperature dependence. Also the film resistance after heating exhibits reciprocal thickness dependence at a constant temperature. The reciprocal thickness dependence of electrical resistivity observed has been explained by the "effective mean free path model" of classical size effect. A reduction in conduction activation energy with increase in film thickness is accounted for by the fact that the grain size increases with thickness, and hence, the barrier height decreases.