We used biodegradable dissolved organic carbon (BDOC) incubations, specific ultraviolet absorbance (SUVA254, indicator of aromatic carbon content) and laboratory experiments to determine the bioavailability and chemical composition of dissolved organic matter (DOM) leached from fresh leaves and litter aged on a seasonally dry floodplain for 2, 4, and 6 months. Our objective was to elucidate how litter age and solar radiation affect the bacterial utilization of DOM released from floodplain leaf litter when inundated. Leachate percent BDOC ranged from 22 to 47% for three different leaf species and significantly decreased (p < 0.05) with increasing litter age. However, total BDOC (mg C L−1) was unrelated to litter age. Bacterial utilization of DOM leachate collected from litter aged on the floodplain for four and six months significantly increased following 48 h of irradiation for all species but there was no difference for leachate from fresh and two month old litter. The photo-mediated increase in percent BDOC was concomitant with a decrease in aromatic carbon content, as SUVA254 values decreased on average 9 ± 6% for light exposure experiments. Our findings demonstrate that sunlight moderates the degradation of plant litter in the terrestrial environment through the photo-mediated shift in DOM composition and its bioavailability in streams.