The capsular polysaccharide produced by Neisseria meningitidis group A has the following structure: [formula: see text] [formula: see text] This polysaccharide was partially hydrolysed with acetic acid, and the oligomers obtained were separated by fast performance liquid chromatography. Six fractions were collected and characterised by ionspray mass spectrometry in the positive ion mode. This soft ionisation technique established the size of the obtained oligosaccharides and the degree of O-acetyl substitution for each fraction.