A high molecular weight poly(propylene oxide-b-ethylene oxide) diblock copolymer was prepared via sequential anionic suspension polymerization using a calcium amidealkoxide initiating system. H-1 nuclear magnetic resonance, viscometry and static and dynamic light scattering have been used to characterize the copolymer and to examine its self-assembly in aqueous solution. The copolymer was found to self-associate in a narrow concentration range above a certain critical aggregation concentration. The weight-average molecular weight, the radii of gyration, the second virial coefficients, the diffusion coefficients, and the hydrodynamic radii of the particles in both unimer and aggregate regions were determined. Aggregates of low aggregation number (2-3) were formed. Dynamic light scattering measurements performed in a wide concentration range revealed an enhanced aggregate stability towards dissociation upon dilution.