Abstract: A process for in situ treatment of a contaminated subsurface zone of the earth containing volatile liquid or semisolid organic contaminants, semivolatile liquid or semisolid organic contaminants and/or nonvolatile organic contaminants in liquid, semisolid and/or solidified form which comprises treating said contaminants with an oxidant such as hydrogen peroxide and conducting vacuum extraction to remove volatilized contaminants, volatilized oxidation products of contaminants and/or liquid oxidation products of contaminants. The oxidant application may be made in a contaminated subsurface vadose zone, a contaminated subsurface capillary fringe or beneath the surface of a contaminated water table. Vacuum extraction may be conducted in the contaminated zone itself or, in the vadose zone directly above the contaminated capillary fringe or the contaminated water table and may be utilized, e.g., to withdraw water containing oxidation products of contaminants.