Phosphorus and silicon are determined sequentially as phosphate and silicate in a single aliquot of solution by an amplification procedure. Phosphomolybdic acid, H3PO4(MoO3)12, is formed and selectively extracted away from silicate and excess of molybdate reagent into isobutyl acetate. After adjustment of the acidity of the remaining silicate solution, silicomolybdic acid, H4SiO4(MoO3)12, is extracted into butanol. The twelve molybdate ions associated with each phosphate and silicate ion are determined by direct atomic-absorption spectroscopy in the isobutyl acetate and butanol phases in a nitrous oxide-acetylene flame at 3132 Å. With this procedure 0·08 to 1 p.p.m. of phosphorus, and 0·08 to 1·2 p.p.m. of silicon can be determined in the same solution. The most suitable conditions for the determination have been established, and a study of the effect of other cations and anions is presented.