Abstract A differential mobility analyzer (DMA) and a Couette centrifugal particle mass analyzer (Couette CPMA) were used to measure the effective density and fractal dimension of particles emitted from a light-duty diesel vehicle fitted with a diesel oxidation catalyst (DOC). It was found that at high engine loads, the DOC increased in temperature, sulphate levels in the particulate matter increased, and a transient nucleation mode was observed. The increase in sulphate levels resulted in a drastic increase in the effective density and fractal dimension of the particles. At low engine loads (8–15%), sulphate levels were much lower, no nucleation mode was present and the fractal dimension varied from 2.22 to 2.48, which is in good agreement with previous studies. At 40% load, sulphate levels were much higher and the fractal dimension was 2.76.