Nitrate acts as an alternative electron acceptor to oxygen during microbial decomposition of organic matter, so has the potential to buffer microbial oxygen demand and reduce anaerobic phosphate release. The effects of nitrate on phosphate release from sediments of an eutrophic estuary were investigated under laboratory conditions, using reconstituted sediment-water cores collected from three sites on three occasions. Application of nitrate to the water column (5-100 mg L^-1 NO3 -N) increased redox potential near the sediment-water interface, from below -200 mV to more than 200 mV during 25 days of incubation. The amount of nitrate required to achieve an effective reduction of soluble reactive phosphate (SRP) in the water column differed between sediments. This was largely a reflection of the differences in sediment properties, particularly bioavailable carbon. Reductions in water column SRP by nitrate application are considered mainly to result from: (1) increased iron (III) binding near the sediment-water interface; and (2) increased DO concentration in the water column due to lowered oxygen demand, and a concurrent increase in oxidation of ferrous iron and SRP binding substances in the water column. High nitrate concentrations (50 and 100 mg L^-1 NO3 -N) did not persist through an entire 155 day incubation, suggesting that if there is no carbon limitation, added nitrate will eventually be consumed by microbial activity, and an increase in phosphate release may occur. Nitrate application directly into the surface sediment increased nitrate consumption, and was therefore less effective, whereas nitrate application to the water column was more effective. The effect of nitrate application was modified by sediment resuspension: frequent resuspension increased DO concentration and therefore reduced nitrate consumption, lowering SRP concentration in the water column.