Abstract: A method for reducing the permeability of a high-permeability zone in a high-temperature oil reservoir comprises:preparing a gellable polymeric aqueous solution, operating on the surface under controlled conditions;injecting said gellable solution into the oil reservoir through a well;moving the solution through the reservoir until it reaches and substantially fills the high-permeability zone to be treated; andgelling the solution in situ with consequent lowering of the permeability in said high-permeability zone, and is essentially characterized in that the gellable solution is an aqueous solution containing a Cr(III) ion cross-linking agent and a water-soluble organic polymer cross-linkable by the effect of said cross-linking agent, said solution being free or substantially free of ligands for the cross-linking agent and having a pH adjusted to a value within the range of 1.5 to 5.5, depending on the desired gelling time.