A numerical model of the time evolution of subsonic aircraft exhaust is used to evaluate the possible activation of soot particles by collisions with SO2 and H2SO4 gas molecules and Brownian coagulation with H2SO4/H2O aerosol formed by homogeneous nucleation. The soluble mass fraction accumulated on soot by the three processes is estimated for emission indices of sulfur from 0.001 to 3 g kg-1. The calculations indicate that the soluble mass fraction of sulfate added to soot particles (assumed to be totally hydrophobic at the point of exhaust) can be large enough to form activated particles within the exhaust plumes of aircraft operating on fuels with typical sulfur contents. However, for emissions from aircraft operating on extremely low sulfur fuels, the soluble material added to soot particles is not sufficient to activate them within the time frame observed for contrail formation. This result, coupled with the Busen and Schumann <1995> observations of contrail formation from an aircraft using 0.004 g S kg-1 fuel, suggests that heterogeneous interactions between soot and sulfur within the exhaust plume are not sufficient to explain the presence of activated particles and contrails in the wakes of high altitude aircraft if the emitted sulfur is in the form of SO2 only. It is probable that soot particles already have enough soluble material when emitted from the engine exhaust, or/and a higher conversion of sulfur into H2SO4 enable them to act as cloud condensation nuclei (CCN) for contrails.¿ 1997 American Geophysical Union