The aim of this work is to study a pre-treatment process of olive mill wastewaters based on ozonation. The efficiency of the process depends on the removal of pollutants and on ozone mass transfer performance. In order to choose an appropriate gas/liquid contactor, the rate constants of three phenolic compounds (gallic acid, p-hydroxybenzoic acid and p-coumaric acid) were determined by using competition kinetic model. These constants, obtained at pH 5. were found to be high (from 3.8 x 10(4) L/mol s to 2.9 x 10(5) L/mol s), inducing a diffusion controlled regime (Ha>3). Thus, to obtain an efficient ozonation process, gas/liquid contactor should be adapted to this regime. An ejector was chosen as gas/liquid contactor. In a first time, treatment of synthetic effluent containing the three phenolic compounds was performed to evaluate efficiency of the process. Experimental conditions were chosen to obtain a diffusion controlled regime (Ha>3). It appeared that this gas/liquid contactor permits obtaining complete and fast removal of pollutants with a very efficient ozone mass transfer (up to 90% during removal of phenolic compounds). So, this process was used to perform the ozonation treatment of olive mill wastewaters from Sfax (Tunisia). It was proved to be very efficient: up to 80% of phenolic compounds were removed and ozone mass transfer reached 95% during this oxidation. (C) 2009 Elsevier B.V. All rights reserved.