A numerical box model of the time evolution of key gaseous and aerosol species in an expanding plume of a subsonic aircraft is developed to investigate the effects of aircraft emissions on the characteristics of contrails. The model simulates the formation and growth of new particles from H2SO4 and H2O and the growth of activated soot particles. Coagulation, dilution, and interaction with gaseous species are included. The evidence of contrail formation is determined by the condensed water concentration, using a visibility threshold of 10-5 kg m-3. The model simulates formation of large concentrations of H2SO4-H2O particles with radii in the range ~1--10 nm. The number concentration and the rate of growth of these particles is dependent on the sulfur emissions. We find that for a given initial concentration of soot particles and ambient conditions the sensitivity of contrail properties to fuel sulfur content is small for typical emission indices (0.1--1 g kg-1). For larger sulfur emissions and favorable conversion of sulfur into H2SO4 the calculations show a slight impact of sulfur on contrail properties, such as lifetime, onset, and threshold temperature. The magnitude of this influence is generally smaller than the variations caused by uncertainties in the mass accommodation coefficient of sulfur species on soot surface, entrainment rate of ambient air into the plume, soot emissions, and atmosphere parameters at flight altitude. ¿ 1998 American Geophysical Union