Thin films of Pb₀.₈Sn₀.₂ Te have been prepared by vacuum evaporation on to heated mica substrates, using a single source technique. Electron microscopy and x-ray diffraction studies have revealed that the films grow with (111) planes parallel to the substrate surface. Further, it was found that continuous films consisted of regions of single orientation being twin related about the [iii] axis. Such regions were identified by the use of dark-field microscopy. Measurements were made of the Hall mobility and Hall coefficient and their variation with temperature, over the range 300°K to 77°K. The temperature dependence of the Hall mobility was found to be consistent with acoustic phonon scattering together with grain boundary scattering. Such observations are similar to those found for the binary lead salt, PbTe. The Hall coefficients for both p-type and n-type Pb₀.₈Sn₀.₂Te films were found to increase with increasing temperature. The Hall coefficient data of the p-type samples has been interpreted in terms of a double valence band model and various parameters have been derived by comparing calculated and experimental data. The Hall coefficient results for the n-type samples have been explained assuming that a varying degree of degeneracy alters the Hall factor over -the temperatures of interest. The effects on the conduction properties of Pb₀.₈Sn₀.₂Te films, by- the gaseous ambients, H and 0₂ were investigated. It was found that H acted as a donor, driving p-type films n-type and that such action could be reversed by the acceptor-like action of 0₂. The results have been analysed in terms of a surface adsorption model and a diffusion model; neither model provides good agreement to the experimental data. Additional chemical action by H has also been found.