A series of engine oil samples collected during a study of their Phosphorus Emission Index 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 to compare these species to those found in both the fresh oil and the residual oil remaining after the volatility test.