In situ electrical conductivity measurements have been made on vacuum-deposited amorphous thin films of various thicknesses of a Se 50Te50 alloy in the temperature range 300-430 K. From the electrical conductivity, x-ray studies, and electron diffraction studies it is found that the as-grown films are amorphous and undergo an irreversible amorphous-crystalline transition on heating in situ. The amorphous-crystalline transition takes place in the temperature range 320-360 K for the different films. However, no systematic variation of the transition temperature with the thickness of the films is observed. All the films except the thinnest ones have a sharp transition temperature. X-ray and electron diffraction analyses show that the Se50Te5 0 films above 360 K are polycrystalline. Above 360 K the electrical conductivity of the polycrystalline Se50Te 50 films varies as an exponential function of reciprocal temperature.