Inherent viscosities were measured for polystyrene (PS) in N,N-dimethylformamide (DMF)/cyclohexane (CH) in the temperature range covered from 65 degrees C close to the upper critical solution temperature T-C of the DMF/CH system. It was found that near the critical temperature, the inherent viscosities contracted to about one-third of the value at 65 degrees C. The overall contraction behavior agrees well with de Gennes＇ theory and some interesting results have been observed. First, in the far off-critical temperature range (log tau > - 1.5), the strong synergetic, co-solvency effect of the mixed solvent was shown by higher values of the inherent viscosity than those measured in any single solvent. Second, the reduced theta temperature, Ts, at which the chain dimension was supposed to have the unperturbed, ideal dimension, weakly depends on the molecular weight. Third, although we observed only the contraction behavior instead of contraction-and-reswelling on approaching the critical point, the expansion factors a(eta)(3)( = [eta]/[eta](theta)) for the different molecular weight samples fell on one master curve when plotted as a function of (log tau - log tau(theta))M-w(1/2). Here tau[ = (T - T-C)/T-C] and M-w are the reduced temperature and the weight-average molecular weight, respectively.