Vapor pressure is the pressure exerted by a vapor on a liquid when it is in equilibrium with its own vapor. Volatility is an expression of vapor pressure or the tendency to evaporate. Loss of oil vapors to the atmosphere (one cause of oil consumption) is directly related to volatility.

Since the vapors of mineral oils are combustible, volatility becomes important in fire safety during the handling and use of oils. The higher the concentration of low molecular weight fractions, the greater the vapor pressure. Vapor pressure is reported as a pressure at a specified temperature. Volatility is reported as percent evaporative weight loss and is measured by ASTM method D-972, where the sample is weighed into a test cell and heated air is passed over the sample for a specified period of time. At the end of the test time the sample is reweighed. The difference between the original sample weight and the after test weight is a measurement of the evaporation loss. Evaporation is reported as percent loss at 250 degree C after 1 hour.