Micelle structural properties were determined for five poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer solutions at 25 degrees C by using light scattering techniques. Size distributions for the Pluronic series P103, P104, P105, F108, and P123 were found from dynamic light scattering. Aggregation numbers from both dynamic light scattering and static light scattering were determined, and the results are interpreted using fundamental hydrodynamic and statistical thermodynamic models. The trends in aggregation behavior are discussed in terms of the surfactant composition. For a fixed poly(propylene oxide) content, surfactants with a higher poly(ethylene oxide) content were found to be less aggregated. For a fixed poly(ethylene oxide)/ poly(propylene oxide) ratio, surfactants with a higher molecular weight were more aggregated. These results are shown to be in agreement with previous self-consistent mean-field calculations for the same series of polymers and are compared with "star" and "brush" model predictions for the effect of poly(ethylene oxide) chain length on the thickness of the micelle corona.