Solids isolated from several different diesel fuels under a variety of conditions of ageing were analyzed in detail by pyrolysis/field ionization mass spectrometry. Using this technique, it was found that filterable and adherent solids appeared to be of similar composition, whereas filterable solids from different fuels appeared to be distinctly different. The effect of ageing the fuels at ambient and under ASTM D5304 conditions resulted in compositionally similar insolubles. A sulfonic acid or a thiophenol added to the fuels tended to produce a filterable insoluble which was compositionally very different from the additive free fuels. The spectra indicate possible involvement of indoles and phenalenes in the higher molecular weight pyrolyzates.