Industrial semirigid and rigid formulations of poly(vinyl chloride) were tested using the cone‐and‐plate Weissenberg Rheogoniometer in the steady shearing mode and the Instron Capillary Rheometer. The viscosity dependence on shear rate showed a discontinuity at a critical shear stress, σ 12, cr , given by log σ 12, cr =4.9±0.2 where the shear stress is in dynes/cm 2 . Both parts of the viscosity‐shear rate curve could be superimposed on master curves according to the theory proposed by Graessley and Segal.