Electrospray ionization mass spectrometry (ESI-MS) was used for the characterization of oligomers which were prepared by ring-opening metathesis polymerization (ROMP) of methyl N-(1-phenylethyl)-2-azabicyclo[2.2.1]hept-5-ene-3-carboxylate (1), employing molybdenum alkylidene initiators of the type Mo(CH(t-Bu)(NAr)(OR)(2) (Ar = 2,6-di(isopropyl)phenyl, R = C(CH3)(3) (I), C10H17 (II), and CCH3-(CF3)(2) (III). ROMP was terminated with acetone/water (99/1, v/v), the solvent also used for ESI-MS. The oligomers (2) had different alkylidene end groups derived from the active species : neopentylidene (from the catalyst), isopropylidene (reaction with acetone) and (2-(methoxycarbonyl)-5-methyl-N-(1-phenylethyl)pyrrolid-3-yl)methylidene (reaction with water, hydrogenation product). Metathesis equilibration and intramolecular reactions of the active species with ester groups, resulting in the formation of cyclic products, were found. The influence of the initiator structure and the reaction time t(R) is discussed.