Increases in surface air temperature are expected to affect microbial activity and carbon cycling in streams. To determine the temperature sensitivity of microbial processes we conducted a study in a reference and an experimentally warmed stream. Both are small, well-shaded headwater streams situated in the Appalachian Mountains, USA and fueled by inputs of coarse particulate organic matter. Following ~1.5 years of baseline monitoring in both streams, one stream was heated for ~3 years. We achieved a 2.3ºC mean increase in temperature (up to ~5°C in summer). We followed microbial responses on naturally occurring leaf litter and small wood collected quarterly (7-22°C interval) by measuring microbial respiration, biomass, growth rate and production of fungi and bacteria, and activity of enzymes involved in carbon sequestration (beta-glucosidase, beta-xylosidase and phenol oxidase). Parameters of activity were scaled per unit of microbial biomass. Respiration rates on leaf litter responded to increases in temperature generally as predicted by the metabolic theory of ecology (MTE), with an apparent activation energy (Ea) of ~0.66 eV, while respiration on wood was less sensitive to warming (0.38 eV). Ea estimates for fungal growth rate ranged from 0.70 to 0.78 eV on leaves, while it showed greater temperature sensitivity at lower temperatures (0.97 eV) than at higher temperatures (0.73 eV) on wood. Similarly, temperature sensitivity of beta-glucosidase and beta-xylosidase was greater on leaves in the reference (1.09 eV) than in the heated stream (~0.57 eV); it was generally low on wood (0.24-0.43 eV). Microbial responses often showed greater temperature sensitivity than predicted by basic MTE assumptions, especially at lower temperatures. Microbial carbon processing in temperate forest streams could be particularly sensitive to warming during the key winter period of high activity following bulk leaf litter input in autumn, which may have important implications for carbon cycling under future climate-change scenarios.