Aggregate-containing solutions of poly(n-hexyl isocyanate) (PHIC) and atactic polystyrene of finite concentrations were studied by static and dynamic light scattering. Their static structure factors, (k) and the intensity autocorrelation functions g((2))(t) exhibited the enhanced low-angle scattering and bimodal relaxation, respectively. From (k) and g((2))(t) obtained, the fast- and slow-mode components of the static structure factor ((S) over cap (k) and (S) over cap (slow)(k)) and of the first cumulant (Gamma(fast) and Gamma(slow)) were estimated as functions of the magnitude of the scattering vector k. As verified by a theory based on the dynamic mean-field theory, if the solution contains only a small amount of aggregates with diffusion coefficients much smaller than that of the nonaggregating component, (S) over cap (slow)(k) and Gamma(fast) give us the osmotic compressibility partial derivativec/partial derivativePi and mutual diffusion coefficient D-m of aggregate-free solutions, respectively, and (S) over cap (slow)(k) and Gamma(slow) provide us the radius of gyration (1/2)(z,A) and hydrodynamic radius R-H,R-A of aggregates, respectively, even at a finite polymer concentration, in a good approximation. In fact, the results of partial derivativec/partial derivativePi and D-m obtained were favorably compared with theories for binary polymer solutions without aggregates. For PHIC solutions, aggregates of lower molecular weight samples had larger (1/2)(z,A) and R-H,R-A, indicating that the aggregates of lower molecular weight PHIC consist of a larger number of polymer chains. Furthermore, the ratio of (1/2)(z,A) to R-H,R-A indicated that aggregates of higher-molecular-weight PHIC are rigid-chain-like but those of lower-molecular-weight PHIC are spherelike.