Static mixers are efficient devices successfully used in water and wastewater treatment such as water ozonation for disinfection and oxidation. The growing interest in new ozone generators delivering high ozone concentrations at low gas flow rates has made static mixers an excellent choice for dissolving ozone into water. However it is still a challenging issue when there is a need to design static mixers for their best process performance. A modelling tool is then required to aid with static mixer design, especially for ozonation processes. In this study, Back Flow Cell Model (BFCM), which is a two-parameter model, was used to simulate at steady state ozone mass transfer from the gas phase to water in Kenics static mixer. Mass transfer resistance developed in the liquid phase controlled the process and was described by a volumetric mass transfer coefficient k(i)a. The model parameters, number of cells (N) and back flow ratio (alpha), were determined from RTD tracer studies and did not change significantly with the gas-to-liquid volumetric flow rate ratios (Q(G)/Q(L)). The model was validated with experimental data and was found to be able to accurately predict the gas and liquid concentrations at the outlet of the mixer alongside ozone transfer efficiency. The BFCM was found reliable, accurate and simple to use. It also offers advantage of being flexible and simple to implement over other models and can be used for efficient static mixer design for ozonation processes. (C) 2010 Elsevier B.V. All rights reserved.