When small concentrations of A-b-B diblock copolymer are mixed with A homopolymer or selective solvent, they often form spherical micelles with cores comprised of the B blocks. Simple mean field calculations predict that relatively low molecular weight B homopolymer can be solubilized within the cores, up to a threshold concentration which depends on molecular weight. These predictions are consistent with electron microscopy studies. In this paper, we present a Monte Carlo study of these systems. The results are in qualitative agreement with the phase behavior predicted by mean field theory and observed in the experiments.