Characterization of block size in poly(ethylene oxide)-b-poly(styrene) (PEO-b-PS) block copolymers could be achieved by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) after scission of the macromolecules into their constituent blocks. The performed hydrolytic cleavage was demonstrated to specifically occur on the targeted ester function in the junction group, yielding two homopolymers consisting of the constitutive initial blocks. This approach allows the use of well-established MALDI protocols for a complete copolymer characterization while circumventing difficulties inherent to amphiphilic macromolecule ionization. Although the labile end-group in PS homopolymer was modified by the MALDI process, PS block size could be determined from MS data since polymer chains were shown to remain intact during ionization. This methodology has been validated for a PEO-b-PS sample series, with two PEO of number average molecular weight (M-n) of 2000 and 5000 g mol-l and M-n(PS) ranging from 4000 to 21,000 g mol(-1). Weight average molecular weight (M-w), and thus polydispersity index, could also be reached for each segment and were consistent with values obtained by size exclusion chromatography. This approach is particularly valuable in the case of amphiphilic copolymers for which M. values as determined by liquid state nuclear magnetic resonance might be affected by micelle formation. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polyrn Chem 47: 3380-3390, 2009