Six different sequences of hexasaccharides, pyridylaminated malto-hexaoses containing one N-acetyl hexosamine (HexNAc) residue, were analyzed using matrix-assisted laser desorption/ionization (MALDI) tandem time-of-flight (TOF) mass spectrometry (MS). Based on the product ion spectra of sodium adducts [M+Na]⁺, the chemical species of the observed product ions contained a HexNAc residue and had high ion abundance, indicating that the HexNAc residue had a higher affinity to sodium atom than glucopyranose. The acetamide group coordinated easily to sodium atom. This general rule of product ion generation was useful to predict the structure of the oligosaccharides based on the MS/MS product ion spectra.