This study is an examination of the sooting behavior of spherical microgravity diffusion flames burning ethylene at atmospheric pressure in a 2.2-s drop tower. In a novel application of microgravity, spherical flames were employed to allow convection across the flame to be either from fuel to oxidizer or from oxidizer to fuel. Thus, spherical microgravity flames are capable of allowing stoichiometric mixture fraction, Z(st), and direction of convection across the flame to be controlled independently. This allowed for a study of the phenomenon of permanently blue diffusion flames-flames that remain blue as strain rate approaches zero. Z(st) was varied by changing inert concentrations such that adiabatic flame temperature did not change. At low Z(st), nitrogen was supplied with the oxidizer, and at high Z(st), it was provided with the fuel. Flame structure, quantified by Z(st), was found to have a profound effect on soot production. Soot-free conditions were observed at high Z(st) and sooting conditions were observed at low Z(st) regardless of convection direction. Convection direction was found to have a smaller impact on soot inception, suppressing formation when convection at the flame sheet was directed towards the oxidizer. A numerical analysis was developed to simulate steady state conditions and aided the interpretation of the results. The analysis revealed that steady state was not achieved for any of the flames, but particularly for those with pure ethylene or oxygen flowing from the porous burner. Furthermore, despite the fact that all flames had the same adiabatic flame temperature, the actual peak temperatures differed considerably. While transient burner heating and burner radiation reduced flame temperature, gas-phase radiative heat loss was the dominant mechanism accounting for these differences. (C) 2003 The Combustion Institute. All rights reserved.