An experimental data base of catalyst conversion efficiency was generated, using a tubular flow reactor which contained either a Pt/Rh (5:1; 40g/ft3) or a Pd/Rh (5:1; 40g/ft3) catalyst sample, for the purpose of updating the kinetic rate constants in the Ford TWC model. Steady-state conversion efficiency of CO, NO, C3H8, C3H6, H2 and O2 through these catalysts were determined for a variety of inlet species concentrations and inlet gas temperatures. These data were obtained for values of redox ratio between 0.5 (excess O2) and 4.0, and inlet gas temperatures between 371°C and 593°C. All experimental details and modeling procedures utilized in obtaining an optimized set of kinetic parameters are included.Results of these experiments show significant improvement in CO and NO conversion efficiency and an increase in NH3 production for both catalyst formulations over previous generation catalyst formulations when redox ratio is greater than unity. These results required the reevaluation of several steady state kinetic parameters prior to any future adjustment of the parameters relating to the transient kinetic mechanism in the non-steady state version of our TWC model. The conversion efficiencies obtained with the Pt/Rh formulation were similar to those obtained with the Pd/Rh formulation over a wide range of conditions; the greatest differences occurred at low temperature or when a high concentration of “slow” burning hydrocarbons (C3H8) was present. Because of this similarity in performance, kinetic rate parameters were generated which describe the “averaged” performance of these two catalyst formulations in addition to the constants which describe the performance of either catalyst formulation.