A colloidal dispersion for chemical mechanical polishing comprising: (a) an abrasive component; and (b) from about 0.05% to about 10% by weight of the abrasive component, a water-soluble amphoteric polymer comprising at least one macromolecular chain B and a part A bonded to a single end of the at least one macromolecular chain B, wherein the macromolecular chain B is derived from one or more ethylenically unsaturated monomers having quaternary ammonium groups or inium groups, and wherein the part A is a polymeric or nonpolymeric group comprising at least one anionic group; wherein the dispersion has a pH of between about 1.5 and about 6. The colloidal dispersion is capable of polishing a substrate comprising silicon nitride and silicon oxide with a reverse selectivity ratio of at least about 27, typically at least 50 the reverse selectivity ratio being the ratio of the rate of removal of the silicon nitride to the rate of removal of the silicon oxide.