A novel method of serine determination during Edman degradation of proteins and peptides which is based on the reaction of serine degradation products with alkyl thiols is described. The physicochemical characteristics and the structures of the reaction products, i.e., of 5-[alkyl-thio)methyl)-3-phenyl-2-thioxo-4-imidazolinones, were determined. The procedure described permits the products of serine degradation to be obtained in a 70-80% yield.