Agricultural vinyl sheet was chosen as a model of flexible polyvinyl chloride (PVC), and samples with and without plastisizer (phthalic acid ester (FAE)) were prepared. Each sample was mixed with NaOH pellets at a different Na/Cl ratio and the mixture was subjected to grinding in air by using a planetary ball mill to investigate the influence of FAE on the mechanochemical (MC) dechlorination reaction between PVC and NaOH. The FAE prevents the MC reaction, but the degree of dechlorination is significantly improved as the Na/Cl molar ratio is increased. The MC reaction itself is independent of FAE, which does not dissolve in the aqueous phase.