Aerosol measurements were carried out in medium-speed diesel engines to determine the aerosol characteristics and formation in four-stroke diesel engines equipped with turbocharger(s) burning heavy fuel and high ash-content heavy fuel oil. The mass size distributions are bimodal with a main mode at 60–90 nm and a second mode at 7–10 μm. The small mode particles are formed by nucleation of volatilized fuel oil ash species, which further grow by condensation and agglomeration. The large mode particles are mainly agglomerates of different sizes consisting of the small particles. The number size distributions peak at 40–60 nm, as also observed in the SEM micrographs. Agglomerates consisting of these primary spherical particles are also found. The TEM micrographs reveal that these particles consist of even smaller structures. Based on the mass and elemental size distributions evidence of high volatility of the fuel oil ash was found. The main effect on the aerosol size distributions was caused by the engine type and fuel oil properties.