Investigations to ascertain the factors and processes affecting the disappearance of waste lubricating oil from soil indicated that under field conditions at Timaru, New Zealand, waste lubricating oil applied at rates of up to 224 t/ha was lost at rates of up to 9.1 t/ha/month as determined by gravimetry of soil extracts. Field and laboratory evidence suggest that oil losses due to surface run-off, sub-soil leaching and evaporation were small and that oil recovery rate was constant for the duration of the field experiment. Increased microbial activity of the oiled soils, the ability of bacteria isolated from such soils to grow rapidly (as determined by large increases in bacterial numbers) in the presence of pure paraffinic oil as a sole carbon source and evidence for immobilization of nitrogen and phosphorus in the oiled soil suggested that microbial action was at least partly responsible for the observed losses. The pattern and increased rates of respiration following addition of pure paraffinic oil to Timaru silt loam under laboratory conditions was evidence that at least part of the observed increase in carbon dioxide production was due to decomposition of the added hydrocarbons rather than mineralization of native soil organic matter. A corresponding decrease in the amount and apparent altered composition of oil present in the treated soil compared with unaltered biologically inactive soil, as determined by gas chromatographic analysis of soil extracts, was considered to indicate that the oil had been modified chemically by biological processes. Average rate of oil disappearance (over 18 months) under field conditions increased with increased oil application rate, a pattern which was repeated for laboratory respiration experiments with Timaru silt loam treated with pure paraffinic oil. Repeated cultivation of the oiled soils increased the rate of disappearance of waste lubricating oil at all application rates and the benefit was most marked for 56 and 224 t/ha oil application rates. Evidence for increased rates of oil decomposition in the presence of added nutrients was provided by an increase in the average rate of oil disappearance following the application of nitrogen to oiled field plots, the lower exchangeable NO₃, NO₂ and NH₃ content of the oiled soils 20 months after oil application, evidence for phosphorus immobilization in the oiled soil and increased inferred average rates of oil mineralization of pure paraffinic oil mixed with Timaru silt loam amended with nitrogen and phosphorus. Results obtained for laboratory soil respiration experiments provided evidence that oil mineralization rates were increased following a second application of oil. Other evidence obtained from soil respiration experiments suggested that the rate of disappearance of waste lubricating oil from Timaru soil was not severely affected by the lead content of the oil. Repeated cultivation of the oiled soils increased the rate of disappearance of waste lubricating oil at all application rates and the benefit was most marked for 56 and 224 t/ha oil application rates. Production of volatiles from unoiled soils was stimulated by the presence of oil. Biological properties of the oiled soil as determined by the shoot dry matter yield of Lolium multiforum Lam. var. Westerwolds "Grasslands Tama" were almost completely restored after twenty months further suggesting biochemical/chemical alteration of the oil. Lead analysis of the plant shoots after the same period indicated increased lead accumulation compared to that of the control. Maximum increase was obtained for 56 t/ha oiled plots while no significant increase was obtained for 224 t/ha suggesting that the higher concentration of oil residues may have suppressed lead uptake.