This study aims to identify the factors that control particulate matter (PM) formation and size distribution in direct-injection spark-ignition (DISI) engines. The test engine used for this research was a 1.6 litre, wall-guided DISI, turbocharged, intercooled, in-line 4 cylinder, Euro IV engine. The exhaust sampling point was before the catalytic converter, i.e. engine-out emissions were measured. The first part of this paper investigates the characteristics of PM number and size distribution of DISI and throttle body injected (TBI) engines. The second part investigates the effect of combustion characteristics of DISI engines on the number of 5nm and 10nm (nucleation) and 200nm (accumulation) PM. A statistical analysis of the coefficient of variance (COV) of the maximum rate of pressure rise (RPmax) over 100 cycles was performed against the COV of 5nm, 10nm and 200nm total particle number. The degree of asymmetry of the COV of RPmax around its mean and the relative peakedness or flatness of the distribution were analysed using Skewness and Kurtosis. It was found that the more positive the skewness and more negative the kurtosis of the COV of RPmax, i.e. a distribution with an asymmetric tail extending towards more positive values, higher RPmax, and a relatively flat distribution, lead to the lowest COV of 5nm (nucleation) particulates. For the 200nm (accumulation) particulates, it was found that a relatively flat distribution and consistent RPmax lead to the lowest COV of particulate number.