Technical Abstract: Dimethyl, diethyl, and di-n-butyl phosphites were reacted with methyl or ethyl oleates using thermally initiated radical reactions. Reactions were conducted with or without the presence of the lauroyl peroxide initiator. The reactions gave a mixture of isomers with the phosphorous attached at the 9 or 10 carbon of the stearates. High yields (94–97%) and high purity products (98-99% by GC) were obtained in the presence of the initiator, while without the initiator, the reaction was very slow resulting in very low conversions (< 50% after 6 days). The phosphonostearate products were positively identified and thoroughly characterized using GC, EI-MS, FTIR, and 1H-, 13C-, and 31P-NMR spectra. GC achieved only partial resolution of the positional isomers. A principal component analysis was applied to successfully separate the MS-EI spectra of fractions from the 9- and 10-isomers. A mechanism to explain the observed MS fragmentation pattern and the relative abundance is proposed. 2D NMR data analysis was applied to assign values of 13C- and 1H-NMR shifts as well as P-C and P-H splitting constants. The molecular volume and the refractive indices of the phosphonostearates were determined experimentally and found to be in agreement with the computationally predicted values using the PM3 semi-empirical method and the group-contribution method of Bondi.