Methoxymethylbenzoguanamine resins were analyzed by matrix-assisted laser desorption/ionization timeof-flight mass spectrometry (MALDI-TOF-MS) and electrospray ionization mass spectrometry (ESI-MS). In the case of MALDI-TOF-MS measurements, they could not be characterized because fragmentation occurred considerably. On the ESI-MS measurement, optimization of needle voltage, cone voltage, and cationizing agent made their molecular-related ions intense with little fragmentation. Cluster ions were formed owing to hydrogen bond among their complex components. Gel permeation chromatography (GPC) was used to fractionate the resins. As a result, di#erent oligomer components could be characterized in detail.