The structure of(ethylene oxide)(26)(propylene oxide)(40)(ethylene oxide)(26) Pluronic P85 block copolymer micelles in the unimer-to-micelle transition region was studied using small-angle neutron scattering. At low concentration (1%), where no intermicellar interactions were evident, aggregation numbers were obtained using the contrast variation method. At higher concentrations a three parameter model accounting for the presence of water in the core and corona of the micelle, and for micelle-micelle interactions, was fitted. The micelle core water content decreases (from 60 to 10%) with increasing temperature in the transition region, with a corresponding increase in the micelle aggregation number (35-62). The micelle core radius remains constant (40 Angstrom). Thus, the broad nature of the unimer-to-micelle transition region may be ascribed to the structural changes which accompany the replacement of micellar core water by polymer.