A laboratory-scale hybrid anaerobic reactor, the bottom third of which was occupied by a sludge blanket, the upper two-thirds by submerged clay rings, was used for treatment of wash waters derived from the purification of olive oil in a new manufacturing process. The reactor was operated under mesophilic conditions at different influent substrate concentrations. The hydraulic retention times (HRT) ranged from 0.20 to 1.02 days under normal operating conditions after the start-up. Chemical oxygen demand (GOD) removal efficiencies of more than 89% were achieved at an organic loading rate(OLR) of 8.0 kg COD/m(3) day. OLR was gradually increased from 2.6 to 7.1 kg COD/m(3) day within 16 days but the anaerobic reactor performances did not change significantly. The reactor was operated under varying influent COD concentrations to test the response of the system to both high and low strength wash waters. The system can tolerate OLRs as high as 17.8 kg COD/m(3) day with an average COD removal efficiency of 76.2%. Although the reactor was fed by diluted influent, with an average COD of 1030 mg/litre, at very high hydraulic loadings (HRT=4.8 h) COD removals over 75% were achieved.