Abstract A kinetic study was carried out on the anaerobic digestion of Olive Mill Wastewater (OMW) and OMW that was previously fermented with Geotrichum candidum, Azotobacter chroococcum and Aspergillus terreus. The bioreactor used was batch fed and contained sepiolite as support for the mediating bacteria. Experimental data observed for methane production ( G) against time ( t) are described by the following equation: G = G M [1 − exp( − Axt S 0 ) , over the COD range studied (3.9–14.5 g/l), where: G M is the maximum methane volume obtained at the end of digestion time; S 0 is the initial substrate concentration; X is the microorganism concentration and A is the kinetic constant of the process, which was calculated using a nonlinear regression. This kinetic parameter was found to be influenced by the pretreatment carried out, and was 4.6, 4.1 and 2.3 times higher for Aspergillus, Azotobacter and Geotrichum-pretreated OMW's than that obtained in the anaerobic digestion of untreated OMW. The kinetic constant increased when the phenolic compound content and biotoxicity of the pretreated-OMW's decreased. Finally, the yield coefficient, T p, was 260 (untreated OMW), 300 ( Geotrichum-pretreated OMW), 315 ( Azotobacter-pretreated OMW) and 350 ( Aspergillus-pretreated OMW) ml CH 4 STP/g COD.