A method of chemical phosphorylation was developed to modify soy protein so as to improve its functional properties. The reaction was carried out by incubating soy protein isolate and cyclic sodium trimetaphosphate in an aqueous solution at pH 11.5 and 35'C for about 3 hours. The reactions ensued were the phosphoesterification of serine residues and the phosphoramidation of lysine residues in soy protein. The phosphorylated soy protein isolate prepared therefrom exhibited much improved functional properties in terms of aqueous solubility, water-holding capacity, emulsifiability and whippability. The nutritive bioavailability of soy protein isolate was not impaired by phosphorylation.