A study of the anaerobic digestibility of two-phases olive mill solid waste (OMSW) was carried out in a laboratory-scale completely stirred tank reactor at mesophilic temperature (35 degreesC)Q. The reactor was operated at influent substrate concentrations of 34.5 g COD/l (OMSW 20%), 81.1 g COD/l (OMSW 40%), 113.1 g COD/l (OMSW 60%) and 150.3 g COD/I (OMSW 80%). The hydraulic retention time (HRT) varied between 40.0 and 8.3 days for the first feed used (OMSW 20%) and between 50 and 10 days for the other three influent substrate concentrations. Chemical oxygen demand (COD) and volatile solids (VS) removal efficiencies of 88.4 and 90.9%, respectively, were achieved at an organic loading rate (OLR) of 12.02 g COD/I day for the most concentrated substrate used (OMSW 80%). The maximum methane production rate was found to be 2.12 1 CH4/l day for the above-mentioned OLR and a HRT of 12.5 days. The system can tolerate OLRs as high as 15.03 g COD/l day with COD and VS removal efficiencies of 82.9 and 86.6%, respectively, for this feed concentration. The methane yield coefficients were 0.30, 0.27, 0.23 and 0.20 1 methane STP/g COD removed for the OMSW concentrations of 20, 40, 60 and 80%, respectively. (C) 2002 Elsevier Science Ltd. All rights reserved.