The vapour pressure and resistivity of liquid selenium have measured as a function of temperature by means of an internally heated pressure-vessel with argon as the pressure-medium. A vapour pressure–temperature relation has been derived for pressures and corresponding temperatures in the ranges 0·013–161·5 atm. and 432·5–1380°. At the lowest pressures, where the vapour should behave ideally, the heat of vaporisation derived is 23·1 kcal. mole–1. The resistivity has been measured over the range 390–1420°. Below 550°, the plot of log ρ against 1/T is linear and confirms semiconductive behaviour. In the range 550–1150°, the resistivity drops more markedly with temperature, but at the highest temperatures the change is diminished. The explanation offered qualitatively is that the selenium changes from a semiconductor to a metallic conductor as the chains in the liquid are thermally destroyed.