Abstract: This paper reports the author's effort to verify some of the present assumptions regarding causes and relationships in the volatility of engine oil phosphorus. Using Selby-Noack volatility data from 1300 engine oils collected by the Institute of Materials (IOM) in three areas of the world, volatilized phosphorus showed very low statistical dependence on either oil volatility or phosphorus concentration in the fresh oil. Rather the data seemed to indicate that the chemistries of the phosphorus-containing additives and their formulation with other additives were the controlling cause of phosphorus volatility and, by extension, emission level. This study permitted the development of a Phosphorus Emission Index that predicts the emission potential of a formulated oil based on the amount of phosphorus found in the volatilized oil collected in the Selby-Noack test.