The kinetics of release of the hydrophobic probes, pyrene and estradiol, from micellar aggregates of poly(ethylene oxide)-b-poly(propylene oxide)-b-(poly(ethylene oxide))-g-poly(acrylic acid) (Pluronic-PAA), have been studied. Release of the probes is bimodal with a fast and a slow component. Diffusion coefficients of 10(-15)-10(-14) cm(2)/s characterize the slow mode of diffusion from the core of the micellar aggregates. The core of the aggregates consists mainly of poly(propylene oxide) (PPO) and possesses different properties from those of bulk PPO, with there being a higher activation energy for diffusion of pyrene and dramatic changes in the polarity of the microenvironment of the probe. The temperature-dependent fine structure of the pyrene spectra (I-3/I-1) in Pluronic-PAA above the critical micellization temperature (cmt) can be explained by a redistribution of pyrene from hydrophobic PPO domains into micelles.