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We show that Ce3+ ions when used as an additive to ceria dispersions enhance plasma-enhanced chemical-vapor-deposited silicon nitride polish or removal rates (RRs). Ceria slurries (0.1 wt% and 140 nm avg. size) containing 2.3 mM Ce(NO3)3 and no other additive gave nitride RRs of [?]300 nm/min at 4 psi and [?]350 nm/min at 5 psi, both at pH4. The nitride RRs measured in the presence of Ce(NO3)3, Ce(CH3COO)3 and KNO3 suggest that the rate enhancement is solely due to the presence of Ce3+ ions. We discuss the underlying mechanism causing high silicon nitride RRs in the presence of Ce3+ ions based on XPS analysis of pre- and post-polished silicon nitride and oxynitride film surfaces, streaming potential data and high oxynitride RRs obtained using the same ceria particle-based slurries as above. It is suggested that the Ce3+ ions convert the upper layers of the nitride film into an oxynitride that is polished by the abrasives at a high rate. In this process, the conversion of the nitride film seems to be the rate controlling step with the oxidation and polishing occurring in a repetitive manner.