Diffusion of a solvent through a polymer is one of the important phenomena in the industry. Such diffusion phenomena, however, have complicated multibody problems, and in general, they are difficult to tightly control. In this study, it was our objective to investigate the effects of the structure of the solvent on the diffusion in polymers, in particular, the effects of the structure on concentration dependence of a mutual diffusion coefficient. Knowledge pertaining to chemical structures and diffusion is very useful for controlling the diffusion phenomena because it allows the design of a solvent possessing the desired diffusion. The diffusion of diphenylsulfide and dibenzothiophene in poly(methyl methacrylate) (PMMA) was investigated. Diphenylsulfide and dibenzothiophene have nearly identical molecular structures and differ in a benzene ring with or without rotation within the molecule. The results indicated that the dependence of the mutual diffusion coefficient was significantly affected by the rotation of the benzene ring present in the molecule.