This paper describes a third-order derivative Spectrophotometric procedure for the determination of traces of phosphate, based on the formation of a ternary ion-association complex by reaction of phosphate with rhodamine 6G in the presence of molybdate. The pink complex is readily formed in 0.08–0.16M H2SO4 medium and remained stable for 2 h. The influence of various instrumental parameters and reaction conditions for maximum colour development are discussed. The few interfering ions can be masked by the addition of appropriate reagents. The procedure is simple, rapid and reliable and allows the determination of 1.5 μg l−1 of phosphate. The relative standard deviation for the determination of 0.08 mg l−1 of phosphate was 1.1%. The method is applicable to the determination of phosphate in river water, sea water, drinking water, milk powder and standard reference materials.