In situ electrical conductivity measurements have been carried out on vacuum-deposited thin films of Se20Te80 alloy during heating and cooling cycles. The electrical conductivity and X-ray diffraction studies show that the as-grown Se20Te80 films are amorphous and, upon heating, undergo an irreversible amorphous-crystalline transition between 315 and 350 K. The observation that the as-grown thin films (deposited at room temperature on glass substrates) are amorphous is in contrast to earlier observations by other workers who find that they are polycrystalline. Above the transition temperature, the electrical conductivity of the polycrystalline Se20Te80 films changes as an exponential function of reciprocal temperature. The amorphous-crystalline transition in Se20Te80 thin films is broad, with films of high initial resistance having lower transition temperatures and low-resistance films having higher transition temperatures. The observation of a broad transition in the case of the present Se20Te80 thin films has to be contrasted with our earlier observations of sharp transitions in the case of Se80Te20 and Se50Te50 thin films. © 1988 Chapman and Hall Ltd.