31P-NMR is a useful tool for identifying and monitoring phosphorous-containing additives. 31P is an NMR active and 100% natural abundance isotope, implying that even a small amount of phosphorus-containing compounds can be detected by NMR. A discussion covers the application of NMR in particular for tribological purpose; and example, illustrating the use of 31P-NMR for tracing zinc bis(dialkyldithiophosphate) in lubricant maintenance. This is an abstract of a paper presented at the Society of Tribologists & Lubrication Engineers Annual Meeting and Exhibition 2012 (St. Louis, MO, 5/6-10/2012).