Atmospheric H<sub>2</sub>SO<sub>4</sub> / H<sub>2</sub>O nucleation influencing effects have been studied in the flow tube IfT-LFT (Institute for Tropospheric Research – Laminar Flow Tube) at 293 ± 0.5 K and a pressure of 1 bar using synthetic air as the carrier gas. The presence of a possible background amine concentration in the order of 10<sup>7</sup>–10<sup>8</sup> molecule cm<sup>−3</sup> throughout the experiments has to be taken into account. In a first set of investigations, ozonolysis of olefins (tetramethylethylene, 1-methyl-cyclohexene, α-pinene and limonene) for close to atmospheric concentrations, served as the source of OH radicals and possibly other oxidants initiating H<sub>2</sub>SO<sub>4</sub> formation starting from SO<sub>2</sub>. The oxidant generation is inevitably associated with the formation of organic oxidation products arising from the parent olefins. These products (first generation mainly) showed no clear effect on the number of nucleated particles within a wide range of experimental conditions for H<sub>2</sub>SO<sub>4</sub> concentrations higher than ~10<sup>7</sup> molecule cm<sup>−3</sup>. Also the early growth process of the nucleated particles was not significantly influenced by the organic oxidation products in line with the expected growth by organic products using literature data. An additional, H<sub>2</sub>SO<sub>4</sub>-independent process of particle (nano-CN) formation was observed in the case of α-pinene and limonene ozonolysis for H<sub>2</sub>SO<sub>4</sub> concentrations smaller than ~10<sup>7</sup> molecule cm<sup>−3</sup>. Furthermore, the findings confirm the appearance of an additional oxidant for SO<sub>2</sub> beside OH radicals, very likely stabilized Criegee Intermediates (sCI). A second set of experiments has been performed in the presence of added amines in the concentrations range of a few 10<sup>7</sup>–10<sup>10</sup> molecule cm<sup>−3</sup> applying photolytic OH radical generation for H<sub>2</sub>SO<sub>4</sub> production without addition of other organics. All amines showed significant nucleation enhancement with increasing efficiency in the order pyridine < aniline < dimethylamine < trimethylamine. This result supports the idea of H<sub>2</sub>SO<sub>4</sub> cluster stabilization by amines due to strong H<sub>2</sub>SO<sub>4</sub>↔amine interactions. On the other hand, this study indicates that for organic oxidation products (in presence of the possible amine background as stated) a distinct H<sub>2</sub>SO<sub>4</sub> / H<sub>2</sub>O nucleation enhancement can be due to increased H<sub>2</sub>SO<sub>4</sub> formation caused by additional organic oxidant production (sCI) rather than by stabilization of H<sub>2</sub>SO<sub>4</sub> clusters due to H<sub>2</sub>SO<sub>4</sub>↔organics interactions.