<p>The formation of organic nitrates (ON) in the gas phase and their impact on mass formation of Secondary Organic Aerosol (SOA) was investigated in a laboratory study for <i>α</i>-pinene and <i>β</i>-pinene photo-oxidation. Focus was the elucidation of those mechanisms that cause the often observed suppression of SOA mass formation by NO<sub>x</sub>, and therein the role of highly oxygenated multifunctional molecules (HOM). We observed that with increasing NO<sub>x</sub> (a) the portion of HOM organic nitrates (HOM-ON) increased, (b) the fraction of accretion products (HOM-ACC) decreased and (c) HOM-ACC contained on average smaller carbon numbers.</p> <p>Specifically, we investigated HOM organic nitrates (HOM-ON), arising from the termination reactions of HOM peroxy radicals with NO<sub>x</sub>, and HOM permutation products (HOM-PP), such as ketones, alcohols or hydroperoxides, formed by other termination reactions. Effective uptake coefficients γeff of HOM on particles were determined. HOM with more than 6 O-atoms efficiently condensed on particles (<i>γ</i></sub><sub>eff</sub> > 0.5 in average) and for HOM containing more than 8 O-atoms, every collision led to loss. There was no systematic difference in <i>γ</i></sub><sub>eff</sub> for HOM-ON and HOM-PP arising from the same HOM peroxy radicals. This similarity is attributed to the multifunctional character of the HOM: as functional groups in HOM arising from the same precursor HOM peroxy radical are identical, vapor pressures should not strongly depend on the character the final termination group. As a consequence, the suppressing effect of NO<sub>x</sub> on SOA formation cannot be simply explained by replacement of terminal functional groups by organic nitrate groups.</p> <p>The fraction of organic bound nitrate (OrgNO<sub>3</sub>) stored in gas-phase HOM-ON appeared to be substantially higher than the fraction of particulate OrgNO<sub>3</sub> observed by aerosol mass spectrometry. This result suggests losses of OrgNO<sub>3</sub> for organic nitrates in particles, probably due to hydrolysis of OrgNO<sub>3</sub> that releases HNO<sub>3</sub> into the gas phase but leaves behind the organic rest in the particulate phase. However, the loss of HNO<sub>3</sub> alone, could not explain the observed suppressing effect of NO<sub>x</sub> on particle mass formation from <i>α</i>-pinene and <i>β</i>-pinene.</p> <p>We therefore attributed most of the reduction in SOA mass yields with increasing NO<sub>x</sub> to the significant suppression of gas-phase HOM-ACC which have high molecular mass and are potentially important for SOA mass formation at low NO<sub>x</sub> conditions.</p>