The mechanisms of volatile organic compound (VOC) emissions from Scots pine (Pinus sylvestris L.) were investigated in laboratory experiments. The plants emitted mainly monoterpenes and acetone. Isoprene was emitted only in small amounts, but the mechanisms of its emissions were similar to those of the other compounds. Isoprene, acetone, and monoterpene emissions from Scots pine could be well described by an algorithm that considers emissions caused by evaporation of VOCs out of pools and emissions in parallel with their biosynthetic production. Monoterpene emissions were mainly affected by temperature. In some cases, monoterpene emissions were also influenced by photosynthetic active radiation implying that monoterpene emissions from Pinus sylvestris occur from storage pools as well as from processes that are linked to monoterpene biosynthesis. The coupling of monoterpene emissions with photosynthesis was confirmed by results of experiments with 13CO2. The 13CO2 exposure resulted in emission of 13C labeled monoterpenes during 13CO2 exposure as well as during the night following the exposure. Similar results were also obtained for isoprene emissions. Scots pine emitted isoprene during illumination as well as in darkness. The emitted isoprene was labeled during 13CO2 exposure and in the night following the exposure. The results obtained for monoterpene emissions in the laboratory were compared to those of outdoor measurements with Scots pine. While the temperature dependencies of emission rates were comparable to those obtained from laboratory experiments, a PAR dependence was not detectable. Temperature variations during outdoor measurements prevented a detection of this dependence. ¿ 2001 American Geophysical Union