Measurements of isoprene and monoterpene emission rates were made from a mature and an immature Eucalyptus globulus tree in a commercial monoculture plantation in central Portugal in 1994 using a branch enclosure sampling system with analysis by gas chromatography/flame ionization and gas chromatography/mass selective methods. Isoprene was the dominant compound emitted and represented over 90% of the total assigned volatile organic compound plant emissions during the day. Other identified species were α-pinene, sabinene, β-pinene, myrcene, limonene, cineole, linalool, α-terpineol, and, tentatatively, cis- and trans-ocimine and an unidentified octatriene. When they were adjusted to standard conditions of temperature (30°C) and light (1000 μmol−2 ms−1) VOC emissions from a 1 year old sapling were 5 times higher (49 and 5.2 μg Cg−1 dry wt h−1 for isoprene and total monoterpenes, respectively) than those from a 7-year-old tree (15 and 0.7 μg C g−1 dry wt h−1 respectively). On a projected leaf area basis these differences were not so apparent (isoprene; 5 and 4 mg m−2 h−1, young and old trees, respectively; monoterpenes, 0.6 and 0.2 mg m−2 h−1, respectively). Emission rates from both trees were closely correlated with incident light flux and temperature, with daytime maxima, and with nighttime minima. Existing models describing emissions in terms of light, temperature, and empirical coefficients were found to adequately predict emissions from the young tree but to grossly overestimate emission rates from the mature tree. This finding has implications for the extrapolation of emission data obtained in the laboratory with immature trees to the canopy, regional, or global scales, although additional measurements are required to determine whether the results presented here can be generalized.