Flame soot samples taken as surrogates for aviation soot have been generated under two limiting conditions of combustion in a rich and lean flame resulting in grey and black soot, respectively. Absolute mean surface residence times τs of H2O adsorbed on soot ranging from 400 to 150 ms have been measured for black soot at 243K and for grey soot at 193K, respectively. The kinetic parameters for desorption of H2O adsorbed on grey and black soot are Ea = 7 ¿ 0.5 and 9 ¿ 0.6 kcal/mol, logA/s-1 = 8.8 ¿ 0.5 and 8.5 ¿ 0.5, respectively. We conclude that soot aerosol may be coated with H2O molecules in a sulfur-free pathway once the particles have cooled down to 500K in the plume of aviation exhaust.