Thermal and chemical processes occurring in exhaust manifold thermal reactors have been closely simulated with a computer model. The model has been developed to predict the performance of thermal reactors operating on multicylinder engines in vehicle service. Output predictions include both transient and steady-state response including reactor warmup at steady-state engine operation. Gas and metal temperatures, heat fluxes, and concentrations of selected exhaust components can be followed as a function of time at various locations within the reactor.Studies carried out using the model indicate that, for the chosen set of operating assumptions, combustion of hydrocarbons and carbon monoxide at steady-state operating conditions is limited by imperfect mixing of exhaust gases with auxiliary air. Most reaction was found to occur in the exhaust ports and inner chamber of the reactor with little additional combustion downstream of these sections.Optimum secondary air injection rates for minimum carbon monoxide survival were predicted for various operating conditions. Transient reactor response to a misfire condition was also followed through use of a model.This paper describes the features of the mathematical model, including description of the FORTRAN programs used. Development of the principles used in formulating the model, including both thermal and chemical considerations, are treated in detail.