A mathematical model of the separation of deuterium from tritium in an electrolytic cell containing D2O and T2O to which lithium was added for ion production is described. The model is compared with the experimental results obtained by measurement of the tritium concentration in the cell and in the recombined off gases. The model yields results that are well within the experimental error. Experimenters can use the model to determine accurately whether any increase in tritium concentration in the cell is due to electrolytic isotope separation. The ratio of the tritium in the cell to the tritium in the recombined off gases was found to be 2. This is in agreement with other work.