Block copolymer micelles have shown high potential as hydrophobic drug carriers. The loading efficiency, partition coefficient, and release profile all play critical roles in micellar drug delivery. As part of a series of studies on these polycaprolactone-block-poly(ethylene oxide) (PCL-b-PEO) micelles in drug delivery, we investigated the solubilization and release of the hydrophobic probes, benzo[a]pyrene and Cell-Tracker CM-DiI (DiI) from these micelles using fluorescence spectroscopy. The same method was also used to determine the partition coefficients of each probe between the core and the exterior solution, which were calculated at different solvent compositions and extrapolated to 100% water. The maximum loading efficiencies of DiI and benzo[a]pyrene were 87% and 32%, respectively. The large difference in the loading efficiency is related to the values of the partition coefficients, which were calculated to be 5800 for DiI and 700 for benzo[a]pyrene. DiI is more highly miscible with the polycaprolactone core compared to benzo[a]pyrene. The release of the hydrophobic probes from the micelles showed a biphasic profile under "perfect sink" conditions; there is an initial burst release, followed by a slow and prolonged release until, eventually, complete release is achieved. The release of the probes from the micelles is under diffusion control as shown by the linearity of the release as a function of the square root of time. Approximate diffusion constants of the order of 10(-15) cm(2)/s for DiI and benzo[a]pyrene were obtained. We demonstrate that the type and the concentration of the incorporated agent influence the loading and the release from PCL-b-PEO micelles. In addition to providing new information on the incorporation and release of benzo [a] pyrene and DiI from these micelles, this study also demonstrates the importance of probe-micelle compatibility in the evaluation of a micellar drug delivery system.