Fuel additives are emitted in the particulate phase, either incorporated in soot particles or forming new ultrafine particles. Formation of new particles occurs as soon as the amount of additive exceeds a certain limit, depending on the soot emission factor. This has been shown by measuring particle size distributions as function of additive concentration and engine power using SMPS analysis. In addition a gravimetric and coulometric analysis has been performed. Finally, filter samples were analyzed by ion coupled plasma mass spectroscopy to investigate residua of the additive in the exhaust. If the additive concentration is increased beyond the onset of particle formation, no further decrease in soot emissions is observed. From this point of view, an additive dosage just at the limit of particle formation seems to be optimal.