C4 perennial grasses are being considered for bioenergy because of their high productivity and low inputs. In side-by-side replicated trials, Miscanthus (Miscanthus x giganteus) has previously been found more than twice as productive as switchgrass (Panicum virgatum). The hypothesis that this difference is attributable to higher leaf photosynthetic rates was tested on established plots of switchgrass and Miscanthus in central Illinois with >3300 individual measurements on 20 dates across the 2005 and 2006 growing seasons. Seasonally integrated leaf-level photosynthesis was 33% higher in Miscanthus than switchgrass (P < 0.0001). This increase in carbon assimilation comes at the expense of additional transpiration since stomatal conductance was on average 25% higher in Miscanthus (P < 0.0001). Whole-chain electron transport rate, measured simultaneously by modulated chlorophyll fluorescence, was similarly 23% higher in Miscanthus (P < 0.0001). Efficiencies of light energy transduction into whole chain photosynthetic electron transport, leaf nitrogen use and leaf water use were all significantly higher in Miscanthus. These may all contribute to its higher photosynthetic rates, and in turn, productivity. Systematic measurement of photosynthesis over two complete growing seasons in the field provides a unique dataset explaining why the productivity of these two species differs and for validating mechanistic production models for these emerging bioenergy crops.