The dynamic behavior of polystyrene (PS) (M(W) = 9.0 x 10(5)), poly(methyl methacrylate) (PMMA) (M(W) = 1.4 x 10(6) or 1.0 x 10(5)), and styrene-methyl methacrylate diblock copolymer (S-b-MMA) (M(W) = 8.0 x 10(4)) mixtures in a solvent isorefractive with PMMA (benzene) was investigated by quasielastic light scattering spectroscopy as a function of temperature and PMMA concentration in the phase separation region. Only two main dynamic modes were present at low PMMA concentrations corresponding to the trace diffusion of PS and S-b-MMA visible probes. In the case of PS homopolymer, the probe diffusion coefficient D-s approximate to c(-2.9), which is close to the reptation prediction for self-diffusion of flexible polymers at Theta-conditions, D-s approximate to c(-3). The concentration dependence of D-c for the trace diffusion of S-b-MMA also follows a power law but with a smaller exponent (D-c approximate to c(-2.3)). With concentrations of PMMA increasing above a threshold concentration, the decay time distribution displayed a complex structure with three main separated bands. The very slow broad mode was related to the phase separation in the mixture and originates from aggregates of probe polymers. The phase separation also appeared on heating in some solutions with appropriate concentration. Experiments have shown that the early stage of the phase separation in the mixtures is mostly due to the aggregation of the high-molecule-weight PS and the low-molecular-weight block copolymer mostly remains molecularly dissolved. Thus, possibly only the late stage of the phase separation process could be influenced by the presence of the diblock copolymer.