To investigate the possibility of structural assignment based on negative-ion MSn spectral matching, four isomers of 2-aminopyridine-derivatized (PA) monosialylated oligosaccharides (i.e., complex type N-glycans with an ?2-3 or ?2-6 linked sialic acid on ?1-6 or ?1-3 antennae) were analyzed by using high-performance liquid chromatography/electrospray ion trap time-of-flight mass spectrometry(HPLC/ESI-IT-TOF MS). The negative ion [M-2H]2- is observed dominantly in the MS1 spectra without the loss of a sialic acid. The MS2 spectra derived from it are sufficiently reproducible that MS2 spectral matching based on correlation coefficients can be applied to the assignment of these isomers. The isomers containing a sialic acid on ?1-6 or ?1-3 antennae can be distinguished by MS2 spectral matching, but the ?2-3 and ?2-6 linkage types of sialic acid cannot be distinguished by their MS2 spectra. However, MS3 spectra derived from fragment ions containing a sialic acid (i.e., C4- and D-type ions) clearly differentiate the ?2-3 and ?2-6 linkage types of sialic acid in their MS3 spectral patterns. This difference might be rationalized in terms of a proton transfer from the reducing-end mannose to the negatively-charged sialic acid. These two moieties are very close in the structural conformations of the precursor C4-type fragment ions, as predicted by molecular mechanics calculations. Thus, negative-ion MSn (n=2, 3) spectral matching was proven to be useful for the structural assignment of these four monosialylated PA N-glycan isomers.