Isoprene plays an important role in regulating the atmospheric trace gas composition, in particular the tropospheric ozone concentrations. Therefore realistic estimates of the seasonal variation of isoprene emission source strengths of strong isoprene-emitting deciduous trees such as pedunculate oak (Quercus robur L.) are required in temperate regions of Europe. In 1995 to 1997 a study was conducted to survey the annual fluctuations of oak isoprene synthase activity and photosynthetic pigment contents, the latter as a parameter for the development of the photosynthetic apparatus of oak leaves. Depending on annual temperature and light profiles (photosynthetic photon flux densities (PPFD)), different seasonal patterns of isoprene synthase activity were observed with maximum activities of 18.4±10.6 nmol m−2 s−1 14.1±5.8 nmol m−2 s−1 and 19.9±7.9 nmol m−2 s−1 in 1995, 1996, and 1997, respectively. On the basis of isoprene synthase activity, chlorophyll a measurements, and phenological data collected from pedunculate oaks of 89 ecological regions covering all of Germany a model was developed for the calculation of the seasonal variation of oak isoprene synthase activity in relation to annual fluctuation of temperature and PPFD. By coupling this model to a numeric process-based isoprene emission model it was possible to predict isoprene emission rates of individual pedunculate oak trees with a deviation of 55%.