The effect of temperature and organic loading rate on the rate of methane production from acidic petrochemical wastewater without neutralization was investigated by continuously feeding an anaerobic upflow fixed-film reactor. The temperatures selected for the studies were 25, 37, 45 and 55 degreesC, Organic loading rate (OLR) for each temperature was varied from 3.6 to 21.7 kg COD m(-3) d(-1). Best performance with respect to COD and BOD reduction, total gas production and methane yield was obtained with the reactor operating at 37 degreesC. OLR could be increased to a maximum of 21.7 kg COD m(-3) d(-1) with 90-95% COD and BOD reduction and methane yield of 0.450 m(3) kg(-1) COD d(-1) added. The reactor operating at 55 degreesC gave the highest methane yield of 0.666 m(3) kg(-1) COD d(-1) at an OLR of 6 kg COD m(-3) d(-1). This decreased to 0.110 m(3) kg(-1) COD d(-1) when the OLR was increased to 18.1 kg COD m(-3) d(-1). The reactor operating at 45 degreesC gave a maximum methane yield of 0.416 m(3) kg(-1) COD d(-1) added at an OLR of 6 kg COD m(-3) d(-1). On further increasing the OLR to 9 kg COD m(-3) d(-1), COD reduction was 89%, however, methane yield decreased to 0.333 m(3) kg(-1) COD d(-1) added. The highest methane yield of 0.333 m(3) kg(-1) COD d(-1) added at an OLR of 6 kg COD m(-3) d(-1) was obtained with reactors operating at 25 degreesC. These studies indicate potential rates of methane production from acidic petrochemical wastewater under different temperatures. This provides a guideline for various kinetic analyses and economic evaluation of the potential feasibility of fermenting acidic wastewater to methane.