A computer model has been written to simulate the Rapid Chloride Permeability Test (RCPT) described in ASTM C1202. The model represents the key processes of diffusion and electromigration using standard equations but then maintains charge neutrality by modelling changes to the voltage distribution. This method enables the model to predict current–time transients similar to those recorded in experiments and it can then be used to obtain basic parameters such as diffusion coefficients for tested samples by optimising to the observed data. Experimental data showing a non-linear voltage distribution is presented together with model results which show that non-linearity has a significant effect on the current. Other predictions from the model are compared with published data and shown to give good agreement.