Dynamic properties of styrene-methyl methacrylate (MMA) statistical copolymer solutions in benzene, a solvent isorefractive with MMA, were studied by dynamic light-scattering spectroscopy (DLS) and by pulsed-held-gradient NMR (PFG-NMR). Two separate dynamic processes were found from the DLS results in the semidilute region. The fast process was related to the cooperation diffusion mode. The slow diffusive process was attributed to the dynamics of composition fluctuations. The diffusion coefficient of the slow process has the magnitude similar to the self-diffusion coefficient determined by PFG-NMR. Dynamic property of the slow "polydispersity" mode were qualitatively interpreted in terms of the theory developed for diblock copolymers.