A CMP oxide slurry includes an aqueous solution containing abrasive particles and two or more different passivation agents. Preferably, the aqueous solution is made up of deionized water, and the abrasive particles are a metal oxide selected from the group consisting of ceria, silica, alumina, titania, zirconia and germania. Also, a first passivation agent may be an anionic, cationic or nonionic surfactant, and a second passivation agent may be a phthalic acid and its salts. In one example, the first passivation agent is poly-vinyl sulfonic acid, and the second passivation agent is potassium hydrogen phthalate. The slurry exhibits a high oxide to silicon nitride removal selectivity.