The value of knowing residence time distributions (RTD's) in mass burn incinerators and the serious limitation imposed by the use of gas volume-flow rate based average residence lime with regard to these incinerators are recognized. The utility for understanding macromixing of having detailed RTD-information about the incinerator overfire region is demonstrated by estimating RTD's using transient numerical simulation of tracer injection into the 2-D, isothermal flow field of a typical mass burn incinerator configuration. Several numerical experiments are conducted to determine the effect of overfire jets on the RTD's of tracer flows introduced into different grate streams. The results indicate that valuable diagnostic information on the state of mixedness in the overfire region can be extracted from these RTD's by mapping them onto a macromixing scale whose extremes are the RTD of a plug flow and that of a stirred tank. Further, they not only underscore the critical role played by jets in achieving desirable stales of macromixing in the overfire region but also point to the significance of the geometry effects. Lastly, the opportunity created by the RTD studies for experimental validation of incinerator overfire mixing simulations is identified.