Abstract: Fast atom bombardment mass spectrometry was applied to the identification of glucosides and galaetosides as n-butaneboronate derivatives. The stereoselective reaction of six glucosides and six galaetosides——1-O-methyl α-D-glucopyranoside, 1-O-methyl α-D-galactopyranoside; 1-deoxy-l-phenyl β-D-glucoside. 1-deoxy-l-phenyl β-D-galactoside; 1-deoxy-p-methoxy-phenyl β-D-glueoside, 1-deoxy-p-methoxy-phenyl β-D-galactoside; 4-methylumbelliferone β-glucoside, 4-methyl-umbelliferone β-galactoside; 4′-hydroxy-flavone-7-β-glucoside, 4′-hydroxyflavone-7-β-galactoside; O-nitrophenyl-β-D-galactopyranoside and puerarin——have been studied by fast atom bombardment mass spectrometry. The characteristic mass spectra of the derivatized glucoside and galactoside showed differences which could be used in the differentiation of the stereoisomers in fast atom bombardment mass spectrometry.