New ultra-low vehicle emission legislation requires advanced catalyst systems to achieve high conversion requirements. Manufacturers have to improve both the washcoat formulations and the catalyst substrate technology to meet these new regulations. This paper will present the results of a computer modeling study on the effects of ultra-thinwall catalysts on hydrocarbon and carbon monoxide light-off performance improvement.The experimental data from catalyst light-off testing on an engine dynamometer are compared with theoretical results of advanced substrate modeling for ultra-thin wall ceramic substrates. Results show that thermal mass has the greatest effect on light-off performance. Decreases in wall thickness offer the greatest benefit to light-off performance by lowering the thermal mass of the substrate, thus allowing it to reach light-off temperature faster. While increasing cell density gives higher surface area, the increase in thermal mass accompanying the ultra-high cell densities can, in some cases, saturate the light-off conversion efficiency. Computer modeling shows the conversion efficiency saturation for substrates with cell densities in excess of 1200 cpsi can be shifted by increasing the PM loading, and achieve better conversion efficiency.