A theory of homogeneous nucleation in a multicomponent vapor is developed by combining classic nucleation and continuous thermodynamics concepts. The perfect gas equation of state is used in conjunction with this theory to obtain a model valid at low vapor pressures. The theory is applied to kerosenes used for fuels in aeronautics (Jet A, JP-7, and RP-1) at temperatures from 220 to 360 K and at vapor pressures up to 1 bar. The results show that although the overall nucleation trends regarding the dependency on vapor pressure and temperature are similar for all kerosenes, Jet A has a distinct behavior compared with JP-7 and RP-1. For all kerosenes, as pressure increases, the nucleus size rapidly decreases and is smallest at low temperatures. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.