A polystyrene-block-poly(ethylene oxide) (PS-b-PEO) copolymer is synthesised in an original manner by nitroxide mediated polymerization of styrene from a PEO macroalkoxyamine. Minor amounts of side products are unavoidably present in the final polymer, including: traces of macroalkoxyamine, PEO-b-PS-b-PEO triblock copolymer-a product of the recombination of PEO-b-PS macro-radical during the initial stage of the polymerization-and hPS-a product of thermal initiation. These species do not represent more than a little percentage of the total composition. They are hardly detectable with classical SEC. We use liquid chromatography at critical conditions (LC-CC), 2D chromatography and LC-CC-NMR to get an exhaustive description of the chemical composition distributions (CCD) and of the molar mass distributions (MMD) of all the minor species quoted above. The results are consistent with the synthesis route and prove the accuracy of the LC-CC based techniques in analysing even very small amounts of species. (c) 2005 Elsevier Ltd. All rights reserved.