The concentration dependence of the reduced viscosity rlsp/c and the translational diffusion coefficient D-t(0) as well as the radius of gyration R-g of polystyrene in toluene were studied by viscometry and laser light scattering (LLS), respectively. The influence of the experimental error on the determination of the rlsp/c was investigated and a quantitative relation was given out. Viscometric experiment found that the rlsp/c-c curve is clearly divided into two parts by dynamic contact concentration c(s). When c > c(s), it shows normal linearity as predicted by Huggins equation. But when c < c(s), i.e., in the extremely dilute regime, the eta(sp)/c-c curve is no longer linear and levels off considering the experimental uncertainty. This new finding was confirmed by the following LLS experiment, which indicates that the size of the polymer chains no longer varies with concentration when c < c(s). As a result, we, for the first time, using the method of combination of viscometry and LLS, prove an important conclusion that in the extremely dilute concentration regime, the reduced viscosity of polymer solution, at least for PS/ toluene, conforming to the Einstein viscosity equation is just the intrinsic viscosity and independent of the concentration. (c) 2006 Wiley Periodicals, Inc.