A flow-injection method is described, in which phosphate standards are introduced into a reagent stream containing Cd(2+) ,resulting in the formation of Cd(3)(PO(4))(2). The associated reduction in free metal concentration is sensed by a cadmium-selective electrode. With the exception of major interference from iodide and moderate interference from bromide and thiocyanate, the system exhibits excellent response to phosphate and selectivity over several common anions in solutions buffered at pH 8.4. A maximum sampling rate of 160/hr is possible for phosphate standards in the concentration range 10(-1)-10(-1)M with a 10(-4)M Cd(2+) reagent stream at a total flow-rate (carrier and reagent stream combined) of 8.4 ml/min.