In situ measurements of total concentration, size distribution, and hydration properties of jet engine exhaust from a range of commercial transports is reported. Significant concentration enhancements (above ambient background) for aircraft exhaust particulates is reported permitting the detection of not only newly formed but also aged plumes, even in the presence of considerable ambient pollution. Two types of particle size distributions are found in the near-field (~8 km behind source) exhaust plume from jet aircraft operating under cruise conditions. One type exhibits the form of the Junge distribution with exponential coefficient -2.4. The second exhibits the Junge distribution form in the small-particle region, below about 50 nm, followed by a larger-particle mode between 0.1 and 0.2 μm. Neither of these observed types of distributions exhibit the sharp drop-off in particle concentrations at the small-particle end of the spectrum that was found in ground-based engine tests. Binary nucleation of sulfuric acid aerosols and heterogeneous nucleation on ion clusters are postulated for particles in this size range. This is supported by the finding of significant numbers of particles having high soluble mass fractions. These data are compared with those taken in ground test cells and those reported by other investigators. ¿ American Geophysical Union 1996