Description: Ocean acidification (OA) and organic matter enrichment (due to coastal eutrophication) could act in concert to shift coral reef carbonate sediments from a present state of net calcification to a future state of net dissolution, but no studies have examined the combined effect of these stressors on sediment metabolism and dissolution. This study used 22-hour incubations in flume aquaria with captive sediment communities to measure the combined effect of OA and organic matter (OM) enrichment, on coral reef sediment gross primary productivity (GPP), respiration (R), and net calcification (Gnet). Relative to control sediment communities, both OA ( 1000 µatm) and OM enrichment (+ 40 µmol C/L) significantly decreased rates of sediment Gnet by 98% and 15% mmol CaCO3/m**2/h, respectively , but the mechanism behind this decrease differed. The OA-mediated transition to net dissolution was geochemical, as rates of GPP and R remained unaffected and dissolution was solely enhanced by a decline in the aragonite saturation state (Omega arg) of the overlying water column. In contrast, the OM-mediated decline in Gnet was due to a decline in GPP/R, thereby biologically reducing overlying seawater Ωarg due to the increased respiratory addition of CO2. The decrease in Gnet in response to a combination of both stressors was additive (- 10% relative to OA alone) but this decrease did not significantly differ from the effect of OA alone. In this study OA was the primary driver of future carbonate sediment dissolution, but longer-term experiments with chronic organic matter enrichment are required.