Abstract: A method and an apparatus are disclosed for the determination of the density of a light hydrocarbon mixture by thermostatting the light hydrocarbon mixture in a large thermostatted vessel at a temperature at which said mixture is liquid in said thermostatted vessel below a level thereof. The large thermostatted vessel is next connected to at least one small container of known mass and volume and thermostatted at said temperature. Then a portion of the thermostatted light hydrocarbon mixture is transferred from said large vessel to the liquid level to the small container until the small container is filled with the light hydrocarbon mixture in liquid form. Any vapors present within the small container are vented leaving behind a liquid light hydrocarbon mixture. The small container is then disconnected from the large thermostatted vessel and the mass of the small container and the light hydrocarbon mixture is determined.