Reaction of the methyl ester of N-acetyl-beta-D-neuraminic acid methyl glycoside with N-acetylimidazole yielded the corresponding 9-O-acetyl- and 4,9-di-O-acetyl derivatives. The structures of these compounds were confirmed by mass spectrometry and both 1H and 13C NMR spectroscopy. The compounds served as model substances in a comparative study of the rate of periodate oxidation of unsubstituted and of 9-O-acetylated-N-acetyl-neuraminic acids. This reaction was strongly hampered by the presence of the 9-O-acetyl group. The low molar absorbancy coefficient of N-acetyl-9-O-acetylneuraminic acid in the periodic acid/thiobarbituric acid assay can be explained by this retardation.