Volatile and nonvolatile aerosols were measured in the wake of a B757 airliner in flight, in concert with measurements of gaseous SOx and CO2 emissions, while the airplane was burning fuel with a sulfur content of either 72 parts per million by mass (ppmm) or 676 ppmm. The volatile aerosol number density exceeded that of the nonvolatile for both fuels and, while the nonvolatile (soot) component was largely insensitive to the fuel sulfur content, the volatile component depleted the gas-phase sulfur species with a condensed fraction that increased from 6% (low S) to 31% (high S). The large proportion of SOx in the aerosol phase and its nonlinear dependence on fuel sulfur content cannot be explained by known combustion mechanisms and has the potential for significant environmental effects. ¿ 1998 American Geophysical Union