We report oil the monitoring and evaluation of the crossover reaction in ring-opening metathesis polymerization (ROMP) via MALDI methods. ROMP of various monomers using several catalytic systems (first-generation (1) and third-generation Grubbs-type (III)) was investigated with structurally different norbornene monomers derived from (+/-)endo,exo-bicyclo[2,2,1]-hept-5-ene-2,3-dicarboxylic acid-bis-O-methyl ester (monomer A), (+/-)endo,exo-bicyclo[2,2,1]-hept-5-eile-2,3-dicarboxylic acid-bis-O-2,2,6,6-tetramethyl piperidinoxyl-ester (monomer A), (+/-)exo-N-(4,4,5,5.6.6,7,7,7-nonafluoroheptyl)-10-oxa-4-azatricyclodec-8-ene-3,5-dione (monomer D), and the highly strained monomer 2-methyl-2-phenyl-cyclopropene (monomer E). The crossover reactions as well as the polymerization kinetics of the various monomers were Studied in detail, in particular, using matrix-assisted laser desorption ionization mass spectrometry (MALDI). Catalyst III offered access to the synthesis of highly defined block copolymers, generating poly(A-b-T), poly(A-b-D). and poly(A-b-E) diblockcopolymers in high precision, whereas catalyst 1 offered access to the diblockcopolymers poly(A-b-D) and poly(A-b-E). Poly(A) was used as a probe to analyze the crossover reaction, revealing well-defined crossover kinetics in the case of monomers T, D, and E and a Subsequent good polymerization after the crossover reaction in the case of monomer E. The presented system allows a simple evaluation and monitoring of crossover reactions in ROM P-based polymerization reactions.