The principle of the method is maintaining a test portion of the PVC at an agreed temperature in a nitrogen gas stream, absorbing the hydrogen chloride evolved in a given amount of an appropriate solution, and potentiometrically determining the amount of hydrogen chloride evolved in relation to the recorded change in pCl of the absorbing solution. The method is recommended for compounded PVC materials and products only, although it can be used for polymers in powder form under appropriate conditions.