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Results are presented regarding the sintering mechanisms of barium hexaferrite (BaFe12O19) and modification thereof via secondary phase additions. Additional studies are presented concerning the effect of additives and microstructural development on the resultant microwave frequency magnetic and dielectric loss properties. The increasing demands on materials for microwave application has required development of barium hexaferrites to achieve fine grain size (i.e., ≤1 μm), a high degree of c axis alignment, high density and a minimum in dielectric loss. To accomplish such a task one area of interest is that of incorporating secondary phases. Additions of SiO2, Al2O3 and CaCO3 are examined in terms of grain size, magnetic, and dielectric loss properties. Thermal analysis methods revealed a redox reaction associated with the sintering process in barium hexaferrite. The use of yttrium iron garnet (Y6Fe10O24) indicated an enhanced densification behavior in comparison to undoped barium hexaferrite which was associated to a decreased redox reaction temperature. The inclusion of the SiO2, Al2O3, and CaCO3 additions were seen to inhibit the redox reaction as measured via thermal analysis while liquid phase sintering occurred.