The mass spectra of methyl 4-azido-4-deoxypentosides were investigated by high-resolution mass spectrometry and determination of metastable transitions. In addition to compounds having free hydroxyl groups at position 2 and 3, 2,3-di-O-methylsulfonyl, 3-O-acetyl-2-O-methylsulfonyl, 2,3-di-O-benzoyl, 3-O-methy], and 2,3-anhydro derivatives have been studied. Schemes for the fragmentation of these substances are given.