Early studies revealed that anisotropic grain growth in Si3N4 is modified by RE additions, increasing as r RE+3 increases. STEM observations showed that the extent of RE attachment on Si3N4 grain prism faces increased in a similar manner. Theoretical studies show that the adsorption behavior was related to each RE’s preference to migrate to the high nitrogen grain surfaces versus to the high oxygen triple point pockets. Furthermore, both the predicted prismatic adsorption sites and the adsorption strengths for the REs were confirmed by STEM observations. Thus, both the phase transformation and the growth normal to the prism planes are altered by RE adsorption. In addition, this behavior remains the same regardless of the secondary additives (e.g., Al2O3, MgO) used to promote densification. This raises the question of how might additives be used to tailor phase transformations and grain growth in other ceramics.