Particulate matter emitted by diesel engines is mainly formed by soot agglomerates, which are composed of primary particles forming irregular clusters. The primary particles have small variations in size and shape, although a narrow distribution can be effectively found. Soot agglomerates were collected with a thermophoretic sampling device installed in the exhaust pipe of a direct-injection diesel engine, and the samples were analysed using high resolution transmission electron microscopy. The size distributions of the primary particles have been shown to be quasi-monodisperse. Their mean size was obtained from averaging 10 primary particles per image, and five images per operating mode. The sampling location, the engine speed, the air/fuel ratio and the exhaust gas recirculation were independently modified, and some variations in the primary particle size were observed, mainly when the air/fuel ratio and the engine speed were varied. A time integrated equation has been proposed for estimating the rate of growth of the particles, which provided good fitting to the measured sizes. This equation uses as input the instantaneous cylinder pressure experimental data, as well as the temperature and heat release records obtained from the analysis of the cylinder pressure data (combustion diagnostic).