A comprehensive analytical and experimental study of thermal reactors has been made. The findings have been incorporated into a computer model capable of simulating reactor performance both during warmup and steady-state operation. This paper treats the warmed-up case. Arbitrary reactor configurations may be explored in regard to the steady-state extent of oxidation of hydrocarbons (HC), CO, and H2 with perfect or imperfect mixing and the detrimental effects of heat losses.This experimental and analytical program focused on the Chevrolet 350 in3 engine-Du Pont model V reactor combination. Experimental and calculated results are presented and compared where possible. Analytical models for the Du Pont reactor were developed each of which treated the exhaust ports, core, and annulus differently. An experimental technique was developed which permits an assessment of the completeness of mixing between exhaust and injected air within a thermal reactor and is expected to be useful as a design aid. In addition to oxidation results for CO, H2, and HC as a whole, measurements were made to determine any changes in NO, aldehydes and olefin, paraffin, and aromatic class proportion affected by the thermal reactor.