Fundamental reaction behavior in dechlorination of concentrated monochloroalkanes (methyl, ethyl, n-propyl, and iso-propyl chloride), whose concentrations were set at as high as 15-20 times the solubility at room temperature, was investigated at various hydrothermal conditions under a saturated vapor pressure up to 380 degreesC. The optimum conditions for the dechlorination treatment and solvent effects on the reactions have been discussed based on the results obtained under specific conditions of temperature, reaction time, stirring rate, solvent states (e.g., volume ratio of chloride to solvent, concentration of additive alkali or alcohol, filling ratio of solution in a vessel) on the dechlorination ratio. All the chlorides studied in the present work were easily dechlorinated by hydrolysis, and the alkyl groups were preserved under hydrothermal conditions in the "subcritical" region. These results indicate the optimum conditions for hydrothermal dechlorination and that the products can be controlled in the yield and ether formation by the concentration of the additives in the solvent.