The charged species present in the exhaust of a small single-cylinder spark ignition engine have been studied using spherical and cylindrical Langmuir probes and also by continuously sampling part of the exhaust gases into a mass spectrometer. Peak ion concentrations in the exhaust gases arc of the order of 107–8 ionscm–3although ion densities vary markedly during an engine cycle. The positive ions arc usually hydrates of H3O+, but some have large masses (> 105a.m.u.) and are ultra fine charged soot particles (diam. Ω10 nm) or their precursors. As for the negatively charged species, normally less than 1% of them arc free electrons. There are significant quantities of hydrates of the ions CO3and HCO3. In addition, negatively charged carbonaceous particles with masses up to and exceeding 105a.m.u. are present. One constant feature of the observations made here is that there are slightly more negatively charged large (mass > 105a.m.u.) ions than positively charged ones, The origins of these ions are discussed: their abundances are largely determined by the time interval between the completion of combustion and the moment of measuring ion concentrations. However, the temperatures experienced by the exhaust gases are also important, in that c.g. temperature determines the identity and mobility of the ions, as well as the mechanism for ion recombination. Preliminary computations have been made of the velocities and temperatures of the exhaust gases from such an engine, in order to calculate ion concentrations. The agreement with experimentally measured values is surprisingly good. The dependence of ion concentrations on ignition timing, air-fuel ratio, engine load, octane rating of the fuel and on whether the gasoline is doped with potassium is investigated.