Steady state rates and time lags for binary homogeneous nucleation of sulfuric acid-water aerosols in aircraft wakes are calculated for relative humidities lower than 100%, i.e., for the case in which no visible contrails form. Formation of the sulfuric acid-water aerosol is shown to depend sensitively on the concentration of sulfuric acid derived from engine exhaust. Nucleation of the sulfuric acid-water aerosol is found to be controlled both by the steady state rate and by the time lag on the timescales relevant to contrail formation. Only for the highest gas-phase acid concentrations are the nucleation time lags sufficiently short that steady state conditions pertain.¿ 1997 American Geophysical Union