Steel-mill waste rolling oil (SmWRO) is considered as hazardous substance with high treatment and disposal fees. Anaerobic process could not only transform the hazardous substance into activated sludge, but also generate valuable biogas. This study aimed at studying the biochemical methane potential of SmWRO under inoculum to substrate VS ratios (ISRs) of 0.25, 0.5, 1, 1.5, 2 and 3 using septic tank sludge as inoculum in mesophilic and thermophilic conditions, with blank tests for control. Specific biogas yield (mL/g VSadded), net biogas yield (mL/g VSremoved) and VS removal were analyzed. The ANOVA results indicated great influence of ISR and temperature on studied parameters. ISR of 1.5 at 55 degrees C and ISR of 1.5 and 2 at 35 degrees C were suggested with the highest specific biogas yield (262-265 and 303 mL/g VSadded). Kinetic analysis showed that Gompertz model fit the experimental data best with the least RMSE and largest R-2. (C) 2013 Elsevier Ltd. All rights reserved.