Electrical isolation within a desired circuit requires shallow trench isolation (STI) to separate active regions with tetraethyl orthosilicate (TEOS) filled trenches. STI chemical mechanical planarization (CMP) is used to facilitate bulk oxide removal by selectively targeting TEOS overburden with colloidal suspensions (slurries) containing ceria (CeO₂) nanoparticles. A critical factor in maintaining high removal rates is the presence of CeO₂ surface oxygen vacancies (O_vac); commonly associated with unoxidized Ce³⁺ particles which when filled, change the oxidation state to a less reactive Ce⁴⁺ form. The goal of this work is to utilize modified slurries at various Ce³⁺/Ce⁴⁺ ratios and determine the influence of oxidation state on oxide/nitride removal rates. Characterization of the CeO₂ particles was determined through UV/Visible and fluorescence spectroscopy along with a novel UV/Visible spectroscopic flow cell technique. The modified slurries were then used to polish both TEOS and Si₃N₄ wafers to probe selectivity with respect to various mechanical parameters. Preliminary results show that greater concentrations of Ce³⁺ valence states, in slurries containing a novel reducing agent, result in higher oxide removal rates than the unmodified slurry. Future work will focus on characterization of the post polish effluent to determine the influence of polishing on certain particle properties, such as oxidation state, zeta potential, and size.