The evolution of beta-Si3N4 microstructures is influenced by the adsorption of rare earth (RE) elements at grain surfaces and by the viscosity of the intergranular phases. Theoretical and scanning transmission electron microscopy studies show that the RE atoms exhibit different tendencies to segregate from the liquid phase to grain surfaces and different binding strengths at these surfaces. When combined with MgO (or Al2O3) secondary additions, the rare earth additives are combined in low-viscosity intergranular phases during densification and the alpha- to beta-phase transformation and microstructural evolution are dominated by the RE adsorption behavior. On the other hand, a much higher viscosity intergranular phase forms when the RE2O3 are combined with SiO2. While the RE adsorption behavior remains the same, phase transformation and microstructure are now dominated by Si3N4 solubility and transport in the high liquid phase. By understanding these additive effects, one can develop reinforced microstructures leading silicon nitride ceramics with greatly improved mechanical behavior.