The role of solid solution shells in the densification behavior of ZrB2-MoSi2 ceramics was analyzed for varying ZrB2 starting particle sizes and MoSi2 contents. The formation of core-shell structures in hot-pressed ceramics in the ZrB2-MoSi2 system was confirmed by SEM, TEM and X-ray diffraction analysis. Microstructure analysis established that each ZrB2 core and its (Zr,Mo)B2 solid solution shell were isostructural with no detectable crystallographic misfit. Two single phase (Zr1-XMoX)B2 solid solutions were synthesized by reactive hot pressing to confirm that ZrB2-MoB2 solid solutions obey a quasi-linear Vegard's law. Additional (Si-free) ZrB2-Mo compositions, ZrB2/Mo/ZrB2 sandwich structures, or ZrB2-MoSi2 ceramics (quenched to room temperature) were hot pressed to study a transient liquid phase and its interdependency on ZrB2 powder purity, connectivity of pathways for mass transfer, wettability, or solubility as a function of applied pressure. The solid solution shells were the result of mass transfer by competing solid state mechanisms of surface and grain boundary diffusion during sintering that may have been boosted by a fugitive Si-based transient liquid phase. Mo was incorporated in diffusion-deposited diboride material at particle-particle necks. The volume fraction of solid solution shells and their Mo contents were affected by processing temperatures. In addition, plastic deformation of MoSi2 filled some closed porosity to aid the ceramics in achieving near full density.