Furnace-out emissions of benzene, toluene, ethylbenzene, and the xylenes have been measured for a research-grade, industrial-style diffusion flame burner operating under well-controlled conditions representative of refinery process heaters. Flame structure was also evaluated by means of visible emission imaging. Excess air was varied from 50% to sub-stoichiometric. The fuels were methane and blends of methane, hydrogen, and propane used to simulate the process gas burned in refineries. Each fuel was tailored so that the inlet and boundary conditions on the flame were invariant between fuels-minimizing fluid mechanic differences. The results show emission levels < 3 ppbv for toluene and < 0.5 ppbv for the other species under normal firing conditions. This is in contrast to emissions from 10 to 150 ppbv observed in refinery field tests (WSPA, 1994). They also show an increase in emissions with fuel hydrogen and propane contents. Under sub-stoichiometric conditions, emissions increase by orders of magnitude, with benzene replacing toluene as the dominant emission.