A new, reversible derivatization procedure for reducing oligosaccharides was developed. Reaction of the model compound lacto-N-tetraose (β-D-Gal-(1→3)-β-D-GlcNAc-(1→3)-β-D-Gal-(1→4)-D-Glc) with aqueous ammonium bicarbonate gave the corresponding β-glycosylamine, which was reacted with fluorenylmethoxycarbonyl chloride to give anN-fluorenyl-methoxycarbonyl β-glycosylamine (FMOC derivative). The free oligosaccharide could be recovered in high yield from the FMOC derivative by brief treatment with 15% aqueous ammonia. Complex oligosaccharide mixtures from human milk were derivatized with the new procedure, and excellent separations of the derivatives on straight-phase silica HPLC columns were achieved.