Abstract: A technique for in situ decontamination of an earth formation. High gas temperatures can be achieved at subsurface formations by supplying a fuel at a subsurface location, such as at the bottom of a wellbore. Pressurized air or gas preheated to a temperature of about 700.degree. F.-1200.degree. F. is supplied to the wellbore to ignite the fuel. The combustion heat produced by the fuel further heats the preheated gas to temperatures in the range of 1200.degree. F.-4500.degree., which heated gases are forced out of the borehole into the contaminated formation. Contaminants are thereby volatilized or oxidized and removed from the formation to the surface.