Polyvinyl chloride (PVC) powder was ground with one of two dried alkaline additives, i.e. KOH and NaOH, pellets in air by a planetary ball mill to investigate the dechlorination reaction during the grinding. Grinding of the mixture causes the size reduction of PVC, with causing its mechanochemical reaction with the additive to form partially dechlorinated PVC and chloride (KCl or NaCl), respectively. In both reaction systems, the dechlorination yield increases with an increase in grinding time and amount of additive. The C=C bonds in the PVC ingredient are created in its structure with decreasing molecular weight (MW). The MW of the polymer ground with KOH is higher than that ground with NaOH. The dechlorination speed of the polymer ground with KOH is faster than that ground with NaOH in the initial stage of grinding, but this is reversed during prolonged grinding.