The formation of cloud condensation nuclei in the jet regime of a B-747 airliner at cruise has been investigated by modeling studies. Both the formation of H2O/H2SO4 clusters by homogeneous nucleation and the deposition of water vapor on soot particles activated by the adsorption of gaseous H2SO4, sulfuric acid hydrates, and H2O/H2SO4 clusters were taken into account. H2SO4 has been assumed to be formed only by OH oxidation of SO2 in the plume. Whereas at ambient temperatures between 219 and 224 K the heterogeneous condensation leads to ice particles with average diameters between 3.0 and 1.1 μm for soot emission indices of EI(soot)=(0.05-0.5) g/kg, respectively, no heterogeneous condensation occurs at higher temperatures. Homogeneously nucleated H2O/H2SO4 clusters, on the other hand, have diameters of less than 7 nm and do not contribute to visible contrail formation. Assuming different sulfur emission indices (0.1 g/kg≤EI(SO2)≤10 g/kg), we conclude that the contrail onset is essentially independent of this quantity and that a fractional H2SO4 surface coverage corresponding to a 0.1 monolayer (ML=0.1) must be sufficient to activate the soot particles for H2O uptake, at least for EI(SO2)≥0.5 g/kgfuel. Calculations based on higher threshold values (i.e., 0.1) lead to results which are in disagreement with the onset of contrail formation as deduced by visual observations. Moreover, the present modeling study provides an estimate of the effect of mutual coupling of homogeneous and heterogeneous condensation pathways. ¿ 1998 American Geophysical Union