The ionization efficiency and fragmentation mechanism of methyl methacrylate (MMA) oligomers (3-mersimilar to8-mer) were investigated by using secondary ion mass spectrometry (SIMS) and APCI mass spectrometry (APCI-MS). Protonation and fragmentation mechanisms of MMA oligomers were clarified by using molecular orbital (MO) methods. MMA oligomers were synthesized in anionic polymerization, and the oligomers were fractionated into 3-mersimilar to8-mer using gel permeation chromatography (GPC In SIMS of MMA oligomers (3-mersimilar to8-me), [MH](+), [MH-CH3OH](+), [MH-methyl formate](+) and [MH-2CH(3)OH-methyl formate]+ appeared. The peak intensities of adduct ions [M + Li](+), [M + Na](+) and [M + K](+) increased with the increase of the polymerization degree. The optimized geometries and H+, Li+, Na+ and K+ affinities of MMA monomer (1-mer), dimer (2-mer), and trimer (3-mer) were calculated using the PM3 and ab initio MO methods. The calculated H+, Li+, Na+ and K+ affinities increased in order of 1-mer, 2-mer and 3-mer of MMA. (C) 2002 Elsevier Science B.V All rights reserved.