The yields of NO2 and oxirane have been studied as a function of pressure in the reactions of NO3 with selected unsaturated hydrocarbons. NO2 yields were determined using a flow system with double resonance REMPI-MS technique in the range 1-30 mbar, argon being the buffer gas. The yields of oxiranes were studied using in situ FTIR technique in a static system in the pressure range 30-1000 mbar. With styrene, oxirane formation was also determined by REMPI-MS. The experiments revealed that the NO2 yields decreased strongly with pressure in the case of aliphatic dienes and styrene, whereas only a negligible pressure dependence of NO2 formation was observed with alkenes or cyclic dienes. These results were confirmed for 2,3-dimethyl-2-butene and for isoprene in the FTIR experiments in which the corresponding oxirane yields were measured at up to 1000 mbar of argon. Air as buffer gas reduced oxirane formation. In the reaction of cis-2-butene and trans-2-butene with NO3 similar yields of trans-2,3-dimethyloxirane were obtained. This finding is taken as the most direct evidence for the NO3 radical addition to the carbon double bond in alkenes in the primary reaction step in agreement with previous assumptions.