We report on the ESI-TOF MS investigation of oligomerization and co-oligomerization reactions via ring-opening metathesis polymerization of noncharged monomers. Thus, the monomers 1-4 ((+/-)endo,exo-bicyclo[2,2,1]hept-5-ene-2,3-dicarboxylic acid-bis-O-methyl ester (1), exo-N-(4,4,5,5,6,6,7,7,7-nonafluoroheptyl)-10-oxa-4-azatricyclodec-8-ene-3,5-dione (2), 3-methyl-3-phenylcyclopropene (3), and (+/-)endo,exo-bicyclo[2,2,1]hept-5-ene-2,3-dicarboxylic acid-bis-O-2,2,6,6-tetramethylpiperidinoxyl ester (4)) were investigated with the catalysts 5-8 (Grubbs catalyst first generation (5), Grubbs catalyst third generation (6), Umicore M1 (7), and Umicore M3 (8)) with respect to their crossover chemistries. Monomers 1-4 differ in ring size and substitution patterns and allow to study the monomer reactivities in the order of increasing reactivity for monomer 3 < 4 approximate to 1 < 2. The measured spectra display a significant difference between the reactions conducted with first- and third-generation catalysts with the main fraction being unreacted catalyst species for the first-generation catalysts 5 and 7, while just a small fraction is composed of oligomer and co-oligomer species. A significant reduction of the amount of the catalyst species and an increase in the fractions of oligomer and co-oligomer species are observed for the third-generation catalysts 6 and 8, in accordance with their higher reactivity as compared to the first-generation catalysts. The highest fraction of co-oligomer species is observed for the crossover reactions 1/3 and 1/4. Propagation of the second monomer, however, is only observed in the combinations 1/2 and 1/4 indicative of the higher reactivity of the norbornenes 2 and 4 when compared to the cyclopropene 3, the latter requiring the addition of hydrochloric acid.