The viscometric behavior of polystyrene (PS) in a mixed solvent of toluene/cyclohexane and with varying temperature was thoroughly investigated. It has been found that, in the plot of the reduced viscosity (eta(sp)/C) vs. concentration, the intrinsic viscosity of PS increased in the mixed solvent with higher ratio of toluene/cyclohexane or when measured at higher temperature, indicating that the polymer-solvent interaction increased in the mixed solvent with a higher ratio of toluene/cyclohexane or at higher temperature. According to the theoretical consideration discussed in the paper, the correlation between the intrinsic viscosity of PS at finite concentration, [eta(PS)](C), and the concentration of PS in solution can be obtained by transforming the plot of the reduced viscosity (eta(sp)/C) VS. concentration of PS in the solvent. It is noted that [eta(PS)](C) decreased with an increase in the concentration of PS in solution, indicating that the polymer dimensions decreased in concentrated solution. The concentration-dependent intermolecular excluded volume effect, which can be quantitatively expressed by the parameter b(Y), is believed to be responsible for the results. In good solvent, the polymer coils expand, resulting in an increase of the intermolecular excluded volume effect in solution. As a result, the parameter b(Y) in poor solvent is less than that measured in good solvent. In Theta-solvents, the parameter b(Y) became zero, indicating that [eta(PS)](C) no longer changed with the increase of the concentration of PS in solution and that polymer coils existed in an unperturbed state.