By means of the pulsed-field gradient nuclear magnetic resonance method, the tracer diffusion of homopolymer chains dissolved in the microphase domain of block copolymer mesophase has been studied for two systems : poly(dimethylsiloxane) (PDMS) in polystyrene(PS)-b-PDMS in the presence of d(6)-benzene as a plasticizer, and poly(ethylene glycol) (PEG) in PS-b-poly(hydroxystyrene-g-PEG)-b-PS. Comparing the diffusion coefficient D of the tracers in the block copolymer matrices with the self-diffusion coefficient D-s of the pure tracers, two regimes with a different dependence of D/D-s on the molecular weight M of the tracers have commonly been found in the two systems. In the lower M regime (regime I), the ratio D/D-s increased remarkably with increasing M, whereas in the higher M regime (regime II), the value of D/D-s was similar to 0.15 and was independent of M. This behaviour could be explained by the M dependence of the spatial distribution of the tracer chains in the microphase domain. In regime I, the tracers deeply penetrate into the brushes of the PDMS or grafted PEG with the depth decreasing with increasing M, while in regime II the tracer chains do not penetrate well into the PDMS or PEG brushes, but rather are interposed between the layers.