The performance of the fast lightoff, ultra-short metal monolith (SMM) catalyst technology was evaluated in a laboratory rig capable of simulating automotive exhaust streams. Tests focused primarily on determining the lightoff behavior of the fresh and engine aged catalyst over a wide range of air to fuel ratios (A/F). Experimental results are compared with estimates of mass transfer controlled operation for the geometry of the catalyst substrate. The effects of different space velocities, cycling between rich and lean gas streams, and addition of SO2 are also investigated. Additionally, the effect of thermal aging on the catalyst/substrate interface is discussed.