Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) has been employed for the analysis of poly(methyl methacrylate) (PMMA), poly(methyl acrylate) (PMA), and polystyrene (PSt), which were obtained by ruthenium-mediated living radical polymerization. Proper selection of analysis conditions gave well-resolved spectra of these polymers (one m/z separation). The MS spectra of these polymers showed only one series of peaks separated exactly by the mass of monomers. The observed mass of each peak agreed well with the theoretical one, which possesses one initiator fragment at the alpha -end and one chlorine group (also from the initiator) at the w-end. The observed isotopic distributions were in good agreement with the simulated distribution based on the polymer structure. On the other hand, the spectra of PMMA, PMA, and PSt prepared by conventional radical polymerization with AIBN showed two or three series resulting from bimolecular radical termination and uncontrolled initiation.