Oligo- and polysialic acids (oligo/polySia) are involved in numerous biological processes depending on the chain length, the comprised type of sialic acids, as well as the glycosidic linkages. Here, we describe the determination of the composition, the sequence, as well as the linkages between the sialic acid residues of lactonized oligo/polySia using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS)(/MS) and electrospray-ionization (ESI)-MS((n)).