This paper deals with the numerical simulation of the spray formation and vaporization of an unsteady fuel spray. An extended vaporization model is presented which accounts for heat and species diffusion within the droplets of the spray. To account for high temperature and high pressure conditions, the fuel properties and the boundary conditions at the droplet surface are calculated by an equation of state which includes the effect of gas solubility. The influence on single droplet vaporization is clearly demonstrated.Spray calculations were performed with the improved model, for n-heptane fuel at gas temperatures of 600 and 800 K with pressures of 30 and 50 bar respectively. The results, pertaining to spray penetration, vapour concentration and droplet velocities, are compared with those of the standard well-mixed model and with experimental data. The extended model is also suitable for a binary fuel, consisting of n-decane and α-methylnaphthalene, at extreme ambient conditions (1000 K, 100 bar). It is found that the difference in results for the various spray models is not pronounced due to the high evaporation rates and the wide distribution of droplet sizes.