Ultrafine particle emissions (6-220 nm) of three different residential oil burners were investigated. Both standard extralight (EL) oil and a highly purified EL oil were used with different concentrations of a ferrocene additive. Most emitted particles were in the size range of well below 50 nm in diameter. Depending on the type of burner, the use of highly purified oil led to a reduction of the total particle number concentration by up to two orders of magnitude. One burner was deliberately operated with nonoptimal oxygen supply, simulating a misadjusted or malfunctioning device with abnormal soot emission. In this case, low concentrations of a ferrocene fuel additive have the potential of reducing the total mass of emitted particles by enhancing the soot burnout of larger sized particles (100 nm).