The modelling of weak acid conversion in an EDl cell is described. The model takes into account the solution and resin ion transport numbers, acid dissociation, and ion exchange equilibrium. Simulations, which provide results that agree well with experiments, enable us to evaluate concentrations and conductivities in each position of the ion exchange packed bed, leading to 2D representations. A reduced flow rate Fred, defined as the ratio of the experimental flow rate over the maximum flow rate which would enable 100% conversion, in the case of 100% current efficiency, has been introduced. Steady-state conversion rate appears to have a strong nonlinear dependency on this dimensionless parameter.