Bench-scale tests were conducted to assess the impact of the most representative phenolic compounds present in olive mill wastewater (OMW), two cinnamic acid derivatives (p-coumaric and caffeic acids) and two benzoic acid derivatives (p-hydroxybenzoic and protocatechuic acids), on the kinetics of acetoclastic methanogenesis. Phenolic compounds were added to cultures transferred from an acetate-enriched seed culture reactor. A control without phenolic compound was included as a basis for comparison. Unacclimated cultures were used to minimize the biodegradation of the toxic organic chemicals during the test. A finite-difference, non-linear, least-squares algorithm was used to estimate kinetic parameters by obtaining a best fit of the experimental data to the classical Monod growth and substrate utilization model. Resulting kinetic coefficients revealed substantial changes in both the maximum rate of acetate conversion, k, and the half-velocity coefficient, K-s, when both cinnamic and benzoic acid derivatives were used. The relative effect of the phenolic compound was manifested in a decrease in the value of k or an increase in the value of K-s as the phenolic compound concentration increased. Therefore, mixed inhibition was occurring. In addition, the toxic effects were clearly related to the molecular structure of the phenolic compound in each pair of toxicants studied, the inhibitory impact being greater for the ortho-diphenols (caffeic and protocatechuic acids) in relation to their corresponding monophenolic compounds (p-coumaric and p-hydroxybenzoic acids, respectively).