Trembling aspen (Populus tremuloides) seedlings were exposed to both elevated (720 ppm; ELEV) and ambient (370 ppm; AMB) concentrations of atmospheric CO2 for a 6-month growing season after which senesced leaves were collected and analyzed for differences in chemical composition. Elevated levels of atmospheric CO2 significantly increased total phenolic compounds, lignin levels, and C:N ratios, while decreasing the concentration of foliar nitrogen. ELEV and AMB leaf aggregates were placed into a headwater stream in the autumn of 1999 for 4 months to assess microbial activity, macroinvertebrate colonization, and leaf decomposition rates. Elevated CO2 significantly reduced 30 day microbial community respiration (-36.8%), and percent leaf mass remaining after 30 and 120 days of stream incubation (-9.4% and -13%, respectively). Low resolution of the experimental design for testing macroinvertebrate responses to altered leaves, including the free movement of macroinvertebrates among leaf aggregates, may explain the lack of treatment effect on invertebrate distribution between AMB and ELEV leaves. Elevated CO2-induced increases in leaf litter total phenolic compounds, lignins, and C:N appear to have negative effects on leaf decomposition, especially in the early stages of the decay process where microorganisms play a dominant role.