The synthesis of a novel type of telechelic block copolymer is described : bifunctional poly(styrene-b-isoprene) diblock copolymers were prepared by anionic polymerization using [2-[(N,N-dimethylamino)methyl]phenyl]lithium as the initiator and 1,3-propanesultone as the terminating reagent. Quaternization of the tertiary amino group yields the alpha,omega-macrozwitterionic block copolymer. The applicability of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) for end group analysis is demonstrated. Using analogous bifunctional polystyrene samples, MALDI-TOF-MS enabled us to identify side reactions reducing end functionality in previous synthetic routes to macrozwitterions. The conversion of chain ends further checked by conventional methods such as dye extraction and titration is found to be almost quantitative.