The electrical conductivity of thin films of Se80Te20 polycrystalline alloy vacuum-deposited at room temperature on glass substrates has been studied during in situ heating and cooling cycles. From the electron diffraction of as-grown films it is seen that the studied films are amorphous at room temperature. The electrical conductivity and electron diffraction studies showed that the as-grown amorphous thin films undergo an amorphous-crystalline transition in the temperature range 340 to 360 K. Upon cooling, the films appear to undergo a crystalline crystalline transition around the same temperatures. There does not appear to be any dependence of the amorphous-crystalline transition temperature on the thickness of the films. However, high-resistance films (thinner films) have a well-defined transition temperature while the low-resistance films (thicker films) have a broader transition. The electrical conductivity of polycrystalline Se80Te20 films above 360 K appears to be an exponential function of reciprocal temperature. © 1987 Chapman and Hall Ltd.