This study investigates the effect of the dilution method on the formation of particles by nucleation during cooling. Nucleation in a perforated tube diluter was studied with experiments and models, using dibutylphthalate (DBP) as the nucleating agent. The experiments demonstrate that nucleation tendencies of various DBP concentrations can be observed in the diluter. Modelling studies were carried out using estimates of DBP nucleation rates from the literature. These indicate that nucleation mostly takes place inside the diluter either at the first orifice rings as some DBP vapour is trapped in low temperature regions or in a similar situation in turbulent eddies when high vapour concentration is still present. The complex turbulent dilution system generates broad particle size distributions. (c) 2006 Elsevier Ltd. All rights reserved.