The density of propylene in the liquid and vapor phases was measured over the range 66° to 92 °C. by a refined dilatometer method and at the higher temperatures at the same time by a float method. Good agreement was found between the two methods, the latter being preferable in the neighborhood of the critical temperature. It was found that at temperatures below the critical, no variation in vapor density accompanied a change in the relative volume of the liquid. The data were obtained principally for an investigation to be published simultaneously on discontinuities above the critical temperature.