Samples of an Ah horizon from a Cambisol under oakwood vegetation were artificially contaminated with diesel (at doses of 20, 40, 80, 160 and 400 microl g(-1)) under laboratory conditions. The presence of the contaminant caused a decrease in the microbial biomass and in phosphomonoesterase, beta-glucosidase and particularly, urease activities. In contrast, the basal respiration and mineralization of nitrogen, specifically ammonification, both increased. The microorganisms that survived in the contaminated soil showed increased metabolic activity, as a result of their use of dead microorganisms as a substrate and of a lack of competition. Although the results indicate that the properties related to the degradative capacity of the soils varied differently in response to the contamination, the use of a biochemical quality index (including different biochemical properties), which was designed for estimating the degree of deterioration of the soil, revealed a clear decrease in the biochemical quality of the diesel-contaminated soils.