Homogeneous nucleation of sub-nanometer H2SO4/H2O germs, their growth and freezing probability in the cooling wake of a subsonic jet aircraft at tropopause altitude are investigated. Heteromolecular condensation, water uptake, and coagulation cause a small subset of the germs to grow into nm-sized solution droplets which overcome the Kelvin barrier. These droplets efficiently take up water vapor from the gas phase, dilute rapidly, and may eventually freeze as water ice. However, results discussed for the case of a B 747 airline suggest that under threshold conditions for the onset of contrail formation, a visible contrail is not likely to be produced by this mechanism. The sensitivity of the calculations to uncertainties in the nucleation theory underlines the need for detailed measurements of the particle microphysics in young aircraft plumes. ¿ American Geophysical Union 1995