A study of the semi-continuous anaerobic digestion process of thermally pretreated two-phase olive pomace (TPOP) at 120 degrees C during 180 min was carried out in laboratory-scale completely stirred tank reactors at mesophilic temperature. Organic loading rates (OLRs) in the range of 2.0-7.0 g VS/(L d) were tested after the pretreatment and the breakage of complex organic matter. Total and soluble chemical oxygen demand (CODt and CODs) removal efficiencies decreased slightly between 77.9-71.7% and 92.3-85.0% when the OLR increased from 2 to 5 g VS/(L d). However, when the OLR increased to 7.0, a sudden decrease in process performance was observed. The maximum methane production rate (1.72 L CH4/(L d)) was achieved at an OLR of 4.5 g VS/(L d). This value was 39.8% higher than that obtained in the semi-continuous anaerobic digestion of untreated TPOP at the same OLR. The modified Stover-Kicannon model and the Grau second-order kinetic model were found to be adequate to fit the experimental results of CODt and CODs, respectively. (C) 2016 Institution of Chemical Engineers. Published by Elsevier IV All rights reserved.