Reinvestigation of numerous ring-opening polymerizations by means of MALDI-TOF mass spectrometry has evidenced that cyclic polymers were formed as the only reaction products or, at least, in large fractions. This finding is ascribed to the intermediate formation of difunctional. chains having active end groups that can react with each other. Due to the low concentration of these difunctional chains cyclization is favored over chain extension according to the Ruggli-Ziegler dilution principle. A polymerization mechanism which usually favors the formation of cyclic polymers is the zwitterionic polymerization, but an exception from this rule is known. The following classes of monomers were discussed: a-amino acid, N-carboxyanhydrides (oxazolidine-2,5-diones), dithiolane-2,4-diones, 5,5-dimethyl-1,3,2-dioxathiolan-4-one-2-oxide, salicylic acid O-carboxyanhydride, L-lactide and D,L-lactide, hexamethyl cyclotrisiloxane, and macrocyclic dithiocarbamates.