The conventional reagents for the reductive amination of sugars, ammonium salts or ammonia, require relatively harsh conditions such as high temperatures or high concentrations. In addition, they give substantial amounts of dimeric byproducts. We have developed a method of using benzylamine as a donor to achieve near quantitative amination of reducing oligosaccharides. Benzylamine reacts with reducing oligosaccharides faster and yields less dimeric byproduct than ammonium ion, rendering it especially advantageous for preparative operation. In combination with a heterobifunctional reagent, 5-[N-(2,2-dimethoxyethyl)carbamoyl]pentanoyl azide, [Lee et al. Biochemistry, 28 (1989) 1856-1861], we could couple a nearly maximal number of phosphorylated mannopentaose molecules to ribonuclease A via its primary amino groups.