This paper describes a simple and rapid procedure for determination of traces of phosphate by means of molybdenum blue chemistry. The use of a cost-effective home-made flow cell with a long path length in combination with a light emitting diode (LED) and a photodiode (PD) is demonstrated as a simple absorbance detector for flow-injection analysis. In this method, a sample is injected into the carrier stream through an injection valve and mixed online with mixed reagent (a mixture of molybdate, bismuth, and ascorbic acid in sulfuric acid). The color intensity of the resulting association complex, molybdenum blue, is measured photometrically ( lambda(max) 875 nm). The proposed method can be used to detect phosphate in the range 0.02-4.0 mg x L(-1) and the precision of the proposed procedure is less than 5% at 0.1 mg x L(-1) phosphorus as phosphate. The method has been successfully applied to a variety of natural water samples.