The block length distributions of two block copolymers of alpha-methylstyrene and styrene, of different molecular weights and relative block lengths, were analyzed by matrix-assisted laser desorption/ ionization time-of-flight (MALDI/TOF) mass spectrometry. A method for the treatment of mass spectroscopic data for block copolymers is proposed. The random coupling hypothesis has been verified experimentally and found to hold for this type of triblock copolymers. The experimental distributions have been compared with the Poisson and Schulz-Zimm models modified to accommodate the change of variables. The agreement between the experimental and calculated distributions is very good for the Schulz-Zimm model and fair for the Poisson model. Mass spectrometry proved to be a useful technique providing unique information on the experimental distributions of both constituent parts in a block copolymer. It was confirmed that block copolymers with narrow molecular weight distributions may have broad, complex, and even bimodal composition distributions. The polydispersity factors observed for the individual parts were higher than those for the whole copolymer.