Two continuous stirred tank reactors (CSTRs) each fed with palm oil mill effluent (POME), operated at 37oC and 55oC, respectively, were investigated for their performance under varies organic loading rates (OLRs). The 37oC reactor operated successfully at a maximum OLR of 12.25 g[COD]/L/day and a hydraulic retention time (HRT) of 7 days. The 55oC reactor operated successfully at the higher loading rate of 17.01 g[COD]/L/day and had a HRT of 5 days. The 37oC reactor achieved a 71.10% reduction of chemical oxygen demand (COD), a biogas production rate of 3.73 L of gas/L[reactor]/day containing 71.04% methane, whereas the 55oC reactor achieved a 70.32% reduction of COD, a biogas production rate of 4.66 L of gas/L[reactor]/day containing 69.53% methane. An OLR of 9.68 g[COD]/L/day, at a HRT of 7 days, was used to study the effects of changing the temperature by 3oC increments. The reactor processes were reasonably stable during the increase from 37oC to 43oC and the decrease from 55oC to 43oC. When the temperature was increased from 37oC to 46oC, the total volatile fatty acid (TVFA) concentration and biogas production was 2,059 mg as acetic acid/L and 1.49 L of gas/L[reactor]/day at day 56, respectively. When the temperature was reduced from 55oC to 40oC, the TVFA concentration and biogas production was 2,368 mg as acetic acid/L and 2.01 L of gas/L[reactor]/day at day 102, respectively. By first reducing the OLR to 4.20 g[COD]/L/day then slowly increasing the OLR back to 9.68 g[COD]/L/day, both reactors were restored to stable conditions at 49oC and 37oC respectively. The initial 37oC reactor became fully acclimatized at 55oC with an efficiency similar to that when operated at the initial 37oC whereas the 55oC reactor also achieved stability at 37oC but with a lower efficiency