AbstractResults of measurements of the temperature dependence of the shear viscosity of critical mixtures of polystyrene/cyclohexane and polystyrene/cyclohexane‐d12using a capillary viscometer are reported. The polystyrene had a molar mass ofMw= 4.0 · 105g mol−1(Mw/Mn≤ 1.03). The data are analysed to determine the value of the critical exponent zηof the viscosity and the value of the system specific viscosity parameter Q0. It is found that zηhas a value of zη= 0.044 ± 0.005 which is smaller than the theoretically predicted value (zη(theoretically) = 0.054). The system specific viscosity parameterQ0has the same value for both systems:Q0= (1.2 ± 0.5) · 106cm−1. A Q0value of this order of magnitude leads to non‐critical contributions to the mutual diffusion coefficient near the critical point r