The phase behavior of a low molecular weight (M-W=4000) diblock copolymer of polystyrene and poly(ethylene oxide), PS-PEO, in the bulk as well in aqueous, D2O, solutions has been studied using small-angle neutron-scattering, dynamic and static light-scattering, and theological methods. At low temperature the pure block copolymer forms a lamellar mesophase driven by the crystallization of PEO. The melting point of PEO, T approximate to 64 degrees C, is correspondingly accompanied by an order-to-disorder transition. The melt is typified by single-exponential depolarized correlation functions in the dynamic light-scattering spectra, observed in the temperature range 60 to 100 degrees C. Neutron scattering fails to show concentration fluctuations in the amorphous, disordered phase due to lack of contrast. In aqueous solutions up to roughly 20% polymer concentration, PS-PEO self-associates into spherical micelles with a core size of R-c approximate to 56 Angstrom and an interaction radius R(hs)approximate to 115 Angstrom, corresponding to an aggregation number of 470. The hydrodynamic radius R-h=140 Angstrom obtained from dynamic light scattering is close to but somewhat larger than the interaction radius, as expected. Extended clusters, displaying a small but significant anisotropy, form at above C=10% and coexist with the single spherical micelles. These clusters may be due to association of spherical micelles or micelles of different form attributed to residual aggregates of PS moities.