The invention provides a chemical-mechanical polishing system comprising an abrasive, a carrier, and either boric acid, or a conjugate base thereof, wherein the boric acid and conjugate base are not present together in the polishing system in a sufficient amount to act as a pH buffer, or a water-soluble boron-containing compound, or salt thereof, that is not boric acid, and a method of polishing a substrate using the chemical-mechanical polishing system.