Diffusion-ordered spectroscopy (DOSY)NMRwas successfully used to characterize amphiphilic block copolymers. A triblock copolymer was prepared by ring-opening polymerisation of a lactide using poly(ethylene glycol) as the initiator. The DOSY NMR experiment is revealed to be a useful analytical method to prove the formation of block copolymers. According to the DOSY map, PLA and PEG blocks exhibited the same diffusion coefficient of 5.623 10 10 m2 s 1, consistent with an efficient polymerisation of the lactide. The determination of the critical micelle concentration (CMC) using DOSY NMR experiments has also been reported. The CMC value correlated with those obtained by fluorimetry and static light scattering. The CMC value was found to be 0.11 g L 1. All the results suggest that DOSY NMR is a valuable analytical tool for the polymer community. The good correlation between the results from DOSY, fluorescence and SLS experiments suggests that the DOSY spectroscopy can accurately measure the CMC of amphiphilic copolymer solutions.