The effects of petroleum products (kerosene, diesel, engine oil and petrol) treatment of soil at various sublethal concentrations (0.0%, 0.1%, 0.25%, 0.5%, 1.0%, 1.5% and 2.0%) on oxidative stress markers (lipid peroxidation, superoxide dismutase activity, catalase activity and xanthine oxidase) were studied in cowpea and maize seedlings. The results indicated that the petroleum products caused a significant increase in lipid peroxidation and a significant decrease in the activities of the antioxidant enzymes: Superoxide dismutase, catalase and xanthine oxidase activities. Kerosene had a greater effect on these indicators of oxidative stress than did the other petroleum products. The effects on lipid peroxidation and antioxidant enzymes were more pronounced in cowpea seedlings than in maize seedlings.