Chemical alterations to leaf litter associated with growth under elevated CO2 may impact aquatic ecosystems that rely on terrestrial leaf litter as a carbon source. This study examined how elevated CO2 altered the chemistry and subsequent response of stream microorganisms growing on the leaf litter of three riparian tree species. Quaking aspen (Populus tremuloides), white willow (Salix alba), and sugar maple (Acer saccharum) were grown under ambient (360 ppm) and elevated (720 ppm) CO2 for an entire growing season and senesced leaf litter was incubated in a stream for 80 days. Elevated-CO2 effects on the chemistry of senesced litter were species-specific. Aspen leaves contained higher concentrations of lignin, maples leaves contained higher concentrations of soluble phenolic compounds, and willow leaves contained higher concentrations of carbohydrate-bound condensed tannins. Initially higher concentrations of soluble phenolic compounds in maple leaves were rapidly leached in stream water. However, higher concentrations of carbohydrate-bound tannins in elevated-CO2-grown willow leaves persisted and were correlated with reduced phenol oxidase activities of attached microbiota. Overall, altered leaf chemistry associated with growth under elevated CO2 did not strongly suppress microbial activity during stream incubation. In cases where there was evidence of suppression, it was largely species-specific.