Poly(vinyl chloride) (PVC) powder and commercial rigid PVC pipe were dechlorinated hydrothermally in the presence of alkalis such as ammonia, sodium hydroxide, and potassium hydroxide in a semi-batch flow reactor, with comparison to dechlorination using water alone. Aqueous ammonia was the most effective among these solvents. Dechlorination with aqueous ammonia proceeded in three stages: initial incubation, major dechlorination, and slow dechlorination. In the initial stage the rates were very slow and scarcely affected by temperature or ammonia concentration. In the second stage extensive dechlorination took place, and the rates were affected by temperature and ammonia concentration. These higher rates were found to be significantly influenced by swelling, not alkalinity, when these various alkalis solvents were compared. In the last stage the rates were slower than in the previous stage, and were not significantly dependent on temperature or ammonia concentration.