The ZDDPs were synthesized by Afton Chemical Corporation (Richmond, VA). ZDDP was first patented as a lubricant additive in the 1940s and its synthesis has been reported numerous times in the literature. The laboratory-prepared materials used here followed similar reaction pathways, where the dithiophosphoric acid was prepared using P2S5 and the requisite alcohol, followed by reaction with ZnO to yield the desired ZDDP complex. The ZDDPs were synthesized from single high-purity (≥98 %) alcohols, purchased from MilliporeSigma (St. Louis, MO) and used without further purification. This ensured that high-purity ZDDPs with well-defined chemical structures were tested. Our 1 H-NMR and C-NMR analysis confirmed that six of the ZDDPs contained only the expected alkyl group, while the oct-2-yl ZDDP contained 98 mol.% oct-2-yl and 2 mol.% oct-1-yl. Previous studies have suggested that negligible alkyl rearrangement occurs during ZDDP synthesis. The ZDDPs were mostly (>80 mol.%) in their neutral form, but they did contain a small amount of the basic form. Previous tribology studies have shown that the two forms behave quite similarly. All of the solutions contained a total ZDDP concentration of 800 ppm of phosphorus, which is similar to the treat rate used in commercial lubricant formulations. Strong additive adsorption is important to tribofilm formation under full-film EHL conditions. Previous powder adsorption experiments have shown very similar adsorption behavior for linear primary (n-butyl), methyl-branched primary (i-butyl), and secondary (s-butyl) ZDDPs from nhexadecane onto iron oxide surfaces. Therefore, for most of the ZDDPs studied here, the adsorption behavior is not expected to significantly affect the tribofilm formation rate on steel.