A two-stage heat-release model was developed and applied to both a divided-chamber and an open-chamber diesel engine to determine the fuel burning rates and product temperatures from measured cylinder pressure-time profiles. Measured NO emission levels for several engine operating conditions were used to select the equivalence ratios of the two stages. Combustion in the first stage was taken to occur at a stoichiometric air-fuel ratio, while second-stage combustion was considered to occur at an equivalence ratio below the cylinder-averaged equivalence ratio. An empirical fit for the equivalence ratio of the second stage was determined. Good agreement between the results of this model and the corresponding single-stage model was obtained for heat-release and heat-transfer histories. The computed combustion temperatures for the rich stage were found to be consistently higher (7 to 22% on an absolute scale) than published flame-temperature measurements. Radiation heat-transfer calculations showed that soot radiation accounted for nearly all radiation, and the contribution of radiation heat transfer to the total heat transfer was found to be between 22 and 38%.