Rates of adsorption of zinc di-isopropyldithiophosphate from n-hexadecane on to iron and steel (13 percent Cr) disks have been measured using 65Zn and 14C radiolabelling. Concentration, stirring, and temperature dependence have been examined. A selective rinsing technique has been used to investigate chemisorption which occurs extensively on iron but not at all on the steel. On iron, adsorption occurs with some release of zinc at all temperatures investigated. On the steel, no zinc is lost from the adsorbed layer. Absence of stirring effects indicates that rates are surface rather than diffusion controlled. Results have been compared with those of previous workers.