Tracer diffusion of asymmetric diblock copolymer in mixtures with homopolymers has been investigated as a function of the volume fraction of diblock copolymer and the molecular weight of matrix homopolymer. The depth profile of polydeuteriostyrene-b-2-vinylpyridine (dPS-PVP) diffusing into the mixture of polystyrene-b-2-vinylpyridine (PS-PVP) with polystyrene was measured by forward recoil spectrometry (FRES), and the tracer diffusion coefficient was determined. Secondary ion mass spectrometry (SIMS) was used for depth profiling when better depth resolution (approx. 15 nm) than that of FRES (approx. 80 nm) was necessary. The ordered structure of the mixtures was measured by small-angle X-ray scattering (SAXS). In concentrated mixtures of PS-PVP diblock copolymer with polystyrene which exhibit an ordered spherical domain structure, the diffusion coefficients are independent of the molecular weight of polystyrene and are also very close to the diffusion coefficient in a neat diblock copolymer melt. On the other hand, in dilute mixtures exhibiting a disordered arrangement of spherical copolymer micelles in a polystyrene matrix, the diffusion coefficient is strongly dependent on the molecular weight (P) of the matrix polystyrene. In highly dilute systems, the diffusion coefficient, D, scales as D approx. P-3, which would be expected for Stokes-Einstein diffusion of spherical micelles in a homopolymer whose viscosity scales approximately as P3. The crossover from the 'activated hopping' diffusion of single diblock copolymers (the diffusion mechanism in the neat diblock copolymer melt) to the Stokes-Einstein diffusion of whole spherical micelles occurs at the volume fraction of the transition between the ordered spherical domain structure and the disordered spherical domain structure.