The use of Electron Energy-Loss Spectroscopy in the Scanning Transmission Electron Microscope, for the single particle analysis of ultrafine aerosol particles, has been assessed using an ambient aerosol sample. It has been shown that under what could be considered routine conditions, compositional data are available from particles down to 5 nm in diameter. Estimates of elemental concentrations were shown to be possible down to 1% in some cases for particles of initially unknown composition. Concentrations lower than this were detectable, but not quantifiable. The use of parallel data acquisition led to credible particle analysis times. Although these were still too long for the technique to be useful in routine analysis, it was shown that the technique is highly applicable where compositional information is required on nanometre-sized particles, and time constraints are not critical.