Most modern high-pressure common rail diesel fuel injection systems employ an internal pressure equalization system in order to support needle lift, enabling precise control of the injected fuel mass. This results in the return of a fraction of the high-pressure diesel back to the fuel tank. The diesel fuel flow occurring in the injector spill passages is expected to be a cavitating flow, which is known to promote fuel ageing. The cavitation of diesel promotes nano-particle formation through induced pyrolysis and oxidation, which may result in deposits in the vehicle fuel system. A purpose-built high-pressure cavitation flow rig has been employed to investigate the stability of unadditised crude-oil derived diesel and paraffin-blend model diesel, which were subjected to continuous hydrodynamic cavitation flow across a single-hole research diesel nozzle. Continuous in-situ spectral optical extinction (405 nm) has been employed to identify and determine variations in fuel composition as a function of the cavitation duration. The results of two high-pressure diesel cavitation experiments are reported. The first dealt with the effect of injection pressure on the rate of induced variation in chemical composition of diesel, and concluded that faster degradation of the fuel occurred at higher pressure. The second experiment involved an investigation into the variation in composition occurring in diesel fuel and the paraffin-blend model diesel, subjected to cavitating flow over a longer duration. Observed differences suggest that the high-pressure cavitation resulted in hydrodynamic sono-chemical destruction of aromatics in the diesel, which is believed to lead to carbonaceous nano-particle formation.