In the attachment reaction of poly(ethylene glycol) oligomers, consisting of 7-13 monomers, with Li+ or K+ in matrix-assisted laser desorption/ionization process, Li+-attached oligomer distribution shows larger fraction of small oligomer ions than K+-attached oligomer distribution, while both distributions are similar to each other for poly(propylene glycol) (6-11 monomers), indicating that the chemical structure of chain is essential in the cationization reaction for linear polyglycols.