Abstract A kinetic study on the anaerobic digestion of olive mill wastewater (OMW) was carried out in a semicontinuous, well-stirred fermenter with microorganisms immobilized on clayey supports. Substrate concentrations equal to or higher than 3·5 g COD/litre within the reactor resulted in a methane production rate that was first-order with respect to biodegradable substrate concentration. The apparent rate constant was proportional to the VSS concentration and varied slightly with the nature of the support. The yield coefficient, Yp/s, was 0.26 litres CH4 STP/g COD irrespective of the support used.