Dissolved reactive phosphorus (DRP) was determined as orthophosphate (PO(4)-P) in fresh and saline water samples by flow-injection (FI) amperometry, without and with in-valve column preconcentration. Detection is based on reduction of the product formed from the reaction of DRP with acidic molybdate at a glassy carbon working electrode (GCE) at 220 mV versus the Ag/AgCl reference electrode. A 0.1M potassium chloride solution was used as both supporting electrolyte and eluent in the preconcentration system. For the FI configuration without preconcentration, a detection limit of 3.4 microg Pl(-1) and sample throughput of 70 samples h(-1) were achieved. The relative standard deviations for 50 and 500 microg Pl(-1) orthophosphate standards were 5.2 and 5.9%, respectively. By incorporating an ion exchange preconcentration column, a detection limit of 0.18 microg Pl(-1) was obtained for a 2-min preconcentration time (R.S.D.s for 0.1 and 1 microg Pl(-1) standards were 22 and 1.0%, respectively). Potential interference from silicate, sulfide, organic phosphates and sodium chloride were investigated. Both the systems were applied to the analysis of certified reference materials and water samples.