More stringent emission regulations have forced an overall systems approach to meeting standards in the most effective manner. An important part of the emissions after-treatment system is the substrate. In recent years, thin wall high cell density substrates have become available. These substrates reportedly offer lower thermal mass and better heat transfer properties for faster light-off and better mass transfer properties for increased performance under stabilized conditions.This paper examines the behavior of high cell density substrate systems under a series of test conditions. A comparison of 400/6, 600/4, 600/3 and 900/2 was carried out both on a bench engine for stabilized and light-off conversion, and also under the ECE/EUDC test procedure. Bench engine results showed significant benefits for HC, CO and NOx emissions when using the higher cell density thinwall substrates. Over the ECE/EUDC testing only the NOx advantage appeared to be significant, presumably because of the relatively poor calibration of the test vehicle. In FTP testing, comparison of 900/2 and a 400/6 substrate showed a significant advantage for the 900/2 substrate for HC and NOx emissions across three different aging cycles and on two different vehicles. Simulation results correctly estimate the magnitude of the benefits of the high cell density substrates over the FTP test cycle, and also predict the effects of other potential substrate products.