A laboratory-scale bioreactor was used to investigate the effectiveness of thermobiore-ediation in the destruction of petroleum hydrocarbons in a soil contaminated with used engine oil. The total kjeldahl and ammonium nitrogen concentrations, moisture content, temperature, and total petroleum hydrocarbon (in fuel and lubricant ranges) were monitored. There was an initial lag period of 4 days after which the destruction of hydrocarbon took place resulting in a rise in the bioreactor temperature. The heavy range hydrocarbons (C21-C32) decreased rapidly from 17,000 ppm to 9,000 in the first 20 days (47% reduction at an average rate of 400 ppm/day) and then decreased slowly to less than 5,000 ppm at day 60 (an additional 23% reduction at an average rate of 100 ppm/day). The medium range hydrocarbons (C10-C21) decreased rapidly from 3,000 ppm to 800 ppm in the first 20 days (73% reduction at an average rate of 110 ppm/day) and then decreased slowly reaching 720 ppm by the end of the experiment (an additional 2.6% reduction at an average rate of 2 ppm/day). The results indicated that the nitrogen was not a limiting factor as the C:N ratio remained in the range of 30:1 to 10:1. The temperature appeared to be the limiting factor in the biodegradation of hydrocarbons. The results also showed significant reduction in the degradation rates during the mesophilic stage (24 -35 degrees C), which lasted from day 4 to day 16. The thermophilic stage caused substantial reduction in the moisture content. However, moisture content was not a limiting factor until day 50 when it dropped below 30%. For optimum bioremediation of hydrocarbons system, reactor temperature must be maintained at or below 35 degrees C with moisture content in the range of 45-50%. Under these conditions, a complete destruction of petroleum hydrocarbons would be achieved in 102 days.