Abstract: An apparatus has been designed that allows the measurement of phosphorus volatility, which is important in, among other things, controlling the degradation of exhaust catalysers in automobiles. Following on from the original Noack volatility test, the Selby-Noack apparatus can measure up to 99.9% of volatiles. The apparatus has been used in the present work to assess the relationship between engine oil volatility and engine oil phosphorus volatility (from additives), some reflections on which are presented.