Capillary rheometry is used to characterize the thermal stability of poly(vinyl chloride). By isothermally extruding the polymer melt at constant shear rate, apparent viscosity–time relationships are obtained having three distinct regions: (1) obtainment of equilibrium viscosity, (2) constant viscosity, and (3) rapidly changing viscosity. During regions 2 and 3 the polymer extrudate gradually changes in color from water clear to black. These characteristic regions are related to the temperature of extrusion by Arrhenius plots. An activation energy of 40 kcal./g.-mole was calculated for the decomposition of PVC. Infrared spectroscopy, thermal gravimetric analysis, and intrinsic viscosity experiments are used to corroborate the results of the melt flow data.