Biogas production of three-phase olive mill solid waste (3POMSW) was studied in lab-scale continuously stirred tank reactors (CSTR) at organic loading rates of 1.5-2.0 g VS L-1 d(-1) and temperatures of 35 and 55 degrees C. To acclimate microorganisms to 3POMSW the start-up phase was performed under thermophilic and mesophilic conditions for 40 days. In order to improve the biogas yield, the enzyme MethaPlus (R) was added and its effect was investigated. Results indicate that thermophilic processing has a higher biogas yield by 8% compared to mesophilic conditions, but there was no significant difference in methane yield. Under thermophilic conditions, addition of the enzyme to 3POMSW resulted in a significant increase in methane yield by 10%. Under mesophilic conditions, the enzyme did not have a significant effect on biogas yield or reactor performance. The quality of biogas for all experiments was sufficient. Long-term experimentation indicated the beginning of reactor failure. (C) 2019 Elsevier Ltd. All rights reserved.