Iodine incorporation UV spectroscopy provides evidence that (PEOA-PPOB-PEOA) block copolymers (Pluronics) of related composition will aggregate either cooperatively or independently. Studying binary mixtures of Pluronics, selected from a series of Pluronics containing the same ratio of PEO:PPO (70% PEG, 30% PPO), allowed the effects of varying polymeric mass on the type of interaction observed to be investigated. Preliminary results indicate that the mass ratio of the PPO moieties determines the type of interaction observed.