The automotive thermal reactor may be suitable for controlling the hydrocarbon (HC) and carbon monoxide (CO) emissions from lean-mixture engines. Little is known about the performance characteristics of “lean” reactors, however, since much of the previous research on reactors considered only rich-engine operation. This study screened the effects of six operating variables on lean-reactor performance by means of a fractional factorial experiment. Also, spatial temperature and species concentration distributions within the experimental two-pass reactor were obtained at several operating conditions.Of the variables screened, reactor insulation had the largest effect on emissions performance. Higher engine speed, retarded spark timing, and a less lean air-fuel ratio were also shown to improve reactor performance significantly. Within the range of this study, exhaust flow rate and engine exhaust-port liners did not significantly affect reactor performance.High HC conversion efficiencies were obtained at many combinations of the six variables. CO conversion efficiencies were generally lower, and at several conditions CO was produced within the reactor from the partial oxidation of HC. Internal sampling revealed that most of the HC oxidation occurred in the reactor core, which was both larger and hotter than the surrounding annulus.