This study investigated the role of catalytic converters on tailpipe-out, nano-scale particulates from a Direct-Injection Spark-Ignition (DISI) gasoline engine. The test engine used for this research was a 1.6 litre, wall-guided DISI, turbocharged, intercooled, in-line 4-cylinder, Euro- IV engine. Spectral density of the particles in the exhaust stream before and after the catalytic converter was measured and was split into four size ranges: 5-10nm, 10-50nm, 50-100nm and 100-1000nm diameter for the sake of analysis. These four size ranges represent both nucleation mode (5-50nm) and accumulation mode (50-1000nm) particulates. The study found that for lower engine operating speeds, i.e. 1600rpm, the catalytic converter had a significant effect on the total number of 5-10 and 10-50nm diameter particles. At certain loading conditions the catalytic converter removed up to 60% of the 5-10nm particles and up to 22% of the 10-50nm particles. However, at higher operating speed such as 3200rpm only up to 6% of the 5-10nm and 10-50nm diameter particle ranges was removed. This study also identified that the catalytic converter has a significant effect on the geometric mean diameter (GMD) of the particles. The GMD of the particulate matter (PM) measured after the catalytic converter, Post-Catalyst, was consistently larger than that of Pre-Catalyst.