Abstract: A Process using anaerobic, aerobic, and cometabolizing bacteria to biodegrade halogenated organic compounds in an unsaturated zone lyng between the ground surface and the underlying water table is disclosed. The process includes contacting in situ an electron donor compound with dehalogenating bacteria indigenous in or added thereto in the unsaturated soil zone which is between the ground surface and underlying water table. Further the halogenated component compounds also react with the bacteria and electron donor; thus, stimulating biodegradation while cycling between aerobic and anaerobic as well as cometabolic environments during the process.