Solutions of an incompatible polystyrene (PS)-polyisoprene (PIP) blend dissolved in CHCl3 are investigated by light scattering as a function of total polymer concentration at a fixed composition in the semidilute regime. Two diffusive modes are observed in the intensity-intensity time-correlation function. As the total polymer concentration increases, it is found that the rate constant of the fast mode increases and that of the slow mode as well as the relative amplitude of the fast mode decrease. With the help of a theory developed in this laboratory, we deduce from the light scattering data the self-diffusion coefficients and the Flory-Huggins interaction parameter chi(F) of the two polymers. The self-diffusion coefficients of both polymers decrease as the total polymer concentration increase. Meanwhile, the Flory-Huggins interaction parameter chi(F) between PS and PIP in CHCl3 (equal to 0.017) is nearly independent of polymer concentration. After making corrections for the excluded volume effect between unlike polymer using polymer solution theory, we obtain from chi(F) a net segment-segment interaction parameter chi(F)(0), which is rather close to that obtained from the solvent free dry blend. (C) 2000 American Institute of Physics. [S0021-9606(00)51415-9].