Abstract: During 2002 the concept and principles of the Phosphorus Emission Index (PEI) was introduced. This paper first reviews the background of the benefits and concerns regarding phosphorus additives. In particular, the paper focuses on the two factors until recently assumed important in phosphorus volatility and catalyst contamination -- oil volatility and initial phosphorus concentration. Studies of Selby-Noack data on 1300 oils collected in 1999 and 2000 by the Institute of Materials (IOM) showed the invalidity of the two assumptions and this led to the concept of the PEI. Further studies in conjunction with a field taxi study by the Ford Motor Company to determine correlation of the PEI with catalyst contamination shows not only correlation but also proved that phosphorus volatility was independent of either oil volatility or fresh oil phosphorus levels. Rather, phosphorus volatility was, as earlier predicted, found to be highly dependent on its chemistry and the chemistry of other additives. Engine oil formulation using the PEI technique should markedly reduce phosphorus volatility and resultant catalyst poisoning.