AbstractTranslational diffusion and internal motion have been observed by dynamic light scattering of optically labeled single chains of polystyrene (PS) in a semidilute solution of poly(methyl methacrylate) and benzene for the case in which the dimensionRgof the PS chain is comparable to the correlation length of the matrix solution. The molecular weightMwdependence of the hydrodynamic radiusRhis expressed asRh∼M w0.91, whileRh∼M w0.54in pure benzene. The average linewidthΓfor internal motions (KRg>1) appears to depend on the magnitudeKof the scattering vector approximately asΓ∝ K4at higherKRg(>1), in contrast with the fact thatΓ∝K3approximately forKRg>1 in pure benzene. The scaling law for theKdependence ofΓdoes not hold in low‐molecular‐weight PS owing to theKdepend