Continuous decomposition of ppb level halocarbons in tap water using an electrochemical method was attempted. Using a flow cell in which electrical contact with the column-type Ag impregnated activated carbon electrode was improved, the concentration of chloroform in tap water decreased to about 1 ppb. The concentration of other halocarbons also decreased to below ppb levels. Using a two-stage treatment, the concentration of chloroform decreased to below the ppb level.