In situ measurements of ultrafine aerosol particle emissions were performed at cruise altitudes behind the Deutsches Zentrum f¿r Luft- und Raumfahrt ATTAS research jet (Rolls-Royce/Snecma M45H M501 engines) and a B737-300 aircraft (CFM International 56-3B1 engines). Measurements were made 0.15--20 s after emission as the source aircraft burned fuel with sulfur contents (FSC) of 2.6, 56, or 118 mg kg-1. Particle size distributions of from 3- to 60-nm diameter were determined by using condensation-nuclei-counters with varying lower size detection limits. Volatile particle concentrations in the aircraft plumes strongly increased as diameter decreased toward the sizes of large molecular clusters, illustrating that apparent particle emissions are extremely sensitive to the smallest particle size detectable by the instrument used. Environmental conditions and plume age alone could influence the number of detected ultrafine (volatile) aerosols within an order of magnitude, as well. The observed volatile particle emissions decreased nonlinearly as FSC decreased to 60 mg kg-1, reaching minimum values of about 2¿1017 kg-1 and 2¿1016 kg-1 for particles >3 nm and >5 nm, respectively. Volatile particle emissions did not change significantly as FSCs were further reduced below 60 mg kg-1. Volatile particle emissions did not differ significantly between the two studied engine types. In contrast, soot particle emissions from the modern CFM56-3B1 engines were 4--5 times less (4¿1014 kg-1) than from the older RR M45H M501 engines (1.8¿1015 kg-1). Contrail processing has been identified as an efficient sink/quenching parameter for ultrafine particles and reduces the remaining interstitial aerosol by factors of 2--10 depending on particle size. These and previously published data are consistent with volatile particle emissions of 2.4¿1017 kg-1 independent of environmental conditions, engine type and FSCs ranging between 2.6 and 2700 mg kg-1. There are clear experimental indication that nonsulfate compounds (probably nonmethane hydrocarbons) begin to dominate the volatile particle composition as FSC decreases below ~100 mg kg-1. ¿ 2000 American Geophysical Union