Wax formation in hydrocarbon fluids at low temperatures is one of the harassing problems faced by the petroleum industry. The prevention of wax formation, the production of diesels more resistant at low temperatures and the design of new and better additives requires a good understanding of the crystallisation behaviour of the paraffin molecules. Better experimental techniques to study the wax formation and reliable thermodynamical models, able to predict the solid-liquid equilibrium of petroleum fluids, would help to prevent and overcome many of the problems associated with wax deposition. Here, an equilibrium cell previously used with synthetic mixtures is applied to study the influence of the desulphuration on the low temperature behaviour of a Petrogal diesel. The compositions of the solid and liquid phases in equilibrium and the fraction of solids in partly crystallised solutions were measured between - 5 and - 20 degrees C. The latter was also measured by DSC, The experimental data was modelled using the Predictive UNIQUAC model. It is shown that the model provides an excellent description of the measured phase behaviour for these diesels. The desulphuration of the diesel seems to make it slightly heavier and to increase its wax appearance temperature (WAT).