Global climate change includes concomitant changes in many components of the abiotic flux necessary for plant life. In this paper, we investigate the combined effects of elevated CO2 (720 ppm) and temperature (+2 K) on the phytochemistry of three deciduous tree species. The analysis revealed that elevated CO(2) generally stimulated increased carbon partitioning to various classes of phenolic compounds, whereas an increase in temperature had the opposite effect. The combined effects of both elevated CO2 and temperature were additive, i.e., canceling one another's individual effects. Obviously, the effects of global climate change on leaf chemistry must simultaneously consider both temperature and CO2. If these results are generally applicable, then the counteracting effect of the temperature is likely to play a major role in alpine, boreal, and arctic zones in determining the balance between populations of plants and herbivores.