Continuing of our programe dedicated the behaviour of polystyrene (PS, subscript 2) in mixed solvents(1-5), measurements of the Huggins constant, k, were made at 34.5°C on solutions of PS dissolved in different conditions of solvent and molecular weight of polymer,0-condi Lions were included so that the expansion factor oc could be determined in each case. Viscosity slope constants (K ) calculated according to an expression of Sakai werec 'plotted as a function of Oc3and were compared with the corresponding curves in terms of experimental values of K. An expression (due to Imai) was examined in order to establish a unique value of the slope constant Kunder 0 - condition by an extrapolation procedure. A constant value of K was not found4for PS. 1/2 -1 We tested the equation relating K CPC, to SM oc which we have proposed (1). A unique line under all conditions with reasonable linearity was obtained.