A new and improved composition and method for profile modification of subterranean formations characterized by high reservoir temperatures and/or harsh brine conditions are disclosed. The new and improved profile modification agents comprise a gelable composition comprising:(a) water;(b) a water-thickening and crosslinkable amount of a water-dispersible copolymer comprising from about 1 to about 99 mol % of units derived from an N,N.sup.1 -dialkylacrylamide monomer copolymerized with from about 1 to about 99 mol % of units derived from acrylic acid or a salt thereof; and(c) an amount of a polyvalent metal sufficient to crosslink the copolymer to form a stable gel. The profile modification agents are effective to alter the permeability of preselected portions of an underground formation by forming strong gels which are stable in harsh brine at temperatures of up to about 120.degree. C. The composition and method are particularly adapted for use in enhanced oil recovery operations.Additionally, component (b) can be a water-dispersible terpolymer comprising from about 5 to about 75 mol percent of units derived fron an N,N.sup.1 -dialkylacrylamide, from about 5 to about 35 mol percent of units derived from acrylic acid or salts thereof and from about 10 to about 90 mol percent of units derived from 2-acrylamido-2-methylpropanesulfonic acid or salts thereof.