Abstract: There is disclosed a co-oxidation method and a reagent system of co-oxidation agents for removing from a contaminated zone below the ground surface a water-immiscible organic compound having a density greater than that of water from a contaminated zone below the ground surface, comprising the steps of (a) injecting into the contaminated zone a solution of at least one inorganic permanganate salt, (b) injecting into the zone a cosolvent compatible with the inorganic permanganate salt, (c) causing reaction of the permanganate salt with the water-immiscible organic compound, and (d) extracting co-oxidant from the contaminated zone, thereby diminishing the level of water-immiscible organic compound present in the contaminated zone.