AbstractThe second virial coefficient A2 and the limiting viscosity number [η] were obtained for polystyrene solutions in toluene, in 1.2‐dichloroethane and in methylethyl ketone over a molecular weight interval of 0.33 < Mw · 10−5 < 65 at 35°C from light scattering and viscosity measurements. The relations in the form of α − αn = k1z for the viscosity expansion factor αη and αp − αq = k2z for the expansion factor α deduced from A2 were studied, where m, n, k1, p, q, and k2 are constants independent of the interaction parameter z = (4π)−3/2BA−3M1/2. The constants m, n, and p, q were determined from the molecular weight dependence of [η] and A2. The value of the polymer‐solvent interaction parameter B, with which the constants k1 and k2 were obtained, was determined so that the calculated value of 2A2/N0B goes to unity as z goes to zero. Here, N0 is the AVOGADRO number. The viscosity data at high molecular weight was explained by the relation α − α = 0.85z. The expansion factor deduced from A2 in the good solvent was described by the relation α5 − α2 = 1.05z. A2M1/2A−3α−3 was practically constant over a wide range of z.