The mutual diffusion of poly(styrene-b-2-vinylpyridine) (PS-PVP) diblock copolymer and PVP-PS-PVP triblock copolymer with polystyrene (PS) homopolymer is measured using secondary ion mass spectrometry. The depth profile of the PVP block in bilayers of PS/PS-PVP films after annealing shows a unique diffusive behavior in that layers of spherical domains of block copolymer are swollen by diffusing homopolymers. We determine the rate of bulk flow across the interface between those layers and find an intrinsic diffusion coefficient of the faster diffusant, i.e., polystyrene. When the molecular weight of PS-PVP (MPS-PVP) is higher than that of PS (M-PS), the intrinsic diffusion coefficient agrees well with the self-diffusion coefficient of PS, suggesting that the driving force of the diffusion is mostly translational entropy; interaction between spherical domains (micelles) is negligibly small. When MPS-PVP < M-PS, PS did not diffuse into deuterated PS-PVP but did diffuse into PS-PVP. This result is caused by the enthalpic interaction between PS and deuterated polystyrene (dPS). The intrinsic diffusion coefficient extracted from the experiment with the PS-PVP/PS couple is smaller than the expected self-diffusion coefficient, suggesting that there is an attractive interaction between PS-PVP spheres in a high molecular weight PS matrix. The PVP-dPS-PVP triblock copolymer was swollen only slightly by diffusing low molecular weight PS. The quite different mutual diffusion behavior of diblock copolymer/homopolymer and triblock copolymer/homopolymer couples is attributed to the presence of bridging midblock chains between triblock copolymer domains.