Abstract: A series of engine oil samples collected during a study of their Phosphorus Emission Index (PEI) values were analyzed by 31P Nuclear Magnetic Resonance Spectroscopy. (The PEI analyses themselves were presented in an associated paper {1}. NMR spectra were generated to obtain and explain the mode of formation and identity of the phosphorus-containing species in the volatiles generated during the Selby-Noack volatility test and compare these species to those found in the fresh oil and the residual oil remaining after the volatility test.