The nature of internal diesel injector deposits (IDID) continues to be of importance to the industry, with field problems such as injector sticking, loss of power, increased emissions and fuel consumption being found. The deposits have their origins in the changes in emission regulations that have seen increasingly severe conditions experienced by fuels because of high temperatures and high pressures of modern common rail systems and the introduction of low sulphur fuels. Furthermore, the effect of these deposits is amplified by the tight engineering tolerances of the moving parts of such systems. The nature and thus understanding of such deposits is necessary to both minimising their formation and the development of effective diesel deposit control additives (DCA).The focused ion beam technique coupled with time of flight secondary -ion mass spectrometry (ToF-SIMS) has the ability to provide information on diesel engine injector deposits as a function of depth for both organic and inorganic constituents. Our previous work with this novel technique is unique in that it has shown layering effects in deposits which may be due to the residual fuel either evaporating and leaving residues or being unable to keep insoluble residues in solution during the injection process. As part of our on-going work to understand the nature of field deposits, the aromatic compounds present have been investigated. To help interpret the results for the aromatic structures present, spectra of a model polycyclic aromatic hydrocarbon (PAH), coronene (C24H12), and coal tar pitch (CTP) have been used as a basis to determine the ring structure of internal diesel; deposits. This work confirms the presence of aromatic ring structures of greater than six rings in composition in injector needle carbonaceous deposits.