The formation of H2SO4/H2O particles has been investigated in a laboratory study using a flow tube at atmospheric pressure. H2SO4 was produced in situ applying different formation pathways. Experiments performed in the absence of organics as well as those in the presence of selected organics with low concentration yielded nearly the same results indicating that organics are not implicitly needed in the formation process of atmospheric H2SO4/H2O particles. The number of newly formed particles increased with increasing humidity in the range r.h. = 11–60%. Measurements of particle size distributions at r.h. = 42% revealed a mean particle diameter of ∼2 nm for an H2SO4 concentration of ∼107 molecule cm−3 and for a residence time of 290 sec. From the measured fraction of positively and negatively charged particles it was concluded that ion-mediated nucleation was negligible under the chosen experimental conditions.