Many researchers have reported the measurement of high numbers of emitted particles from gasoline vehicles operating at high speed. To date, in the absence of standard test protocols or analytical techniques, these measurements have all been made from a dilution tunnel set up according to regulatory procedures. Currently, there is great uncertainty relating to the use of the dilution tunnel as a suitable tool for the measurement of automotive particle size and number distribution and also the relevance of the procedure to ambient measurement of the same parameters.Gasoline particle number emissions, as measured on a dilution tunnel, are low at speeds under 120km/h. Beyond this speed, high numbers of very small particles have been measured. There is some evidence to show that these particles may be formed as an artefact within the sampling system, either from the desorption of deposited material or from the pyrolysis of other material in the sampling system itself.This current study, although designed to investigate the composition of gasoline particles, has shown that the effect of temperature, both within the exhaust pipe and in the sampling system, is critical for the measurement of particle numbers. These particles are directly related to the temperature of the exhaust pipe and sampling system and to the subsequent release of deposited material. They do not represent particles emitted directly from the engine. It has been shown that prolonged periods at elevated temperatures can ‘clean-up’ the system for subsequent measurements. These results have serious implications for the measurement of gasoline particle number emissions using dilution tunnels.