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Glycosidic oxocarbenium ions A1+ were formed by isobutane chemical ionization from methyl 2,3,4,6-tetra-O-methyl-beta-D-mannopyranoside, methyl 2,3,4,6-tetra-O-methyl-beta-D-galactopyranoside and methyl 2,3,4,6-tetra-O-methyl-beta-D-glucopyranoside (the ring -O-being converted into -O+=), and their reaction with ammonia was studied by Fourier transform ion cyclotron resonance spectrometry. Very slow formation (reaction efficiency 0.6-1.4%) of the adduct ion [A1 + NH3]+ was observed as the main process for carefully thermalized ions A1+. Interestingly, the efficiency of the adduct ion formation depends on the sterochemistry of ions A1+.