The automotive, aerospace, defense, and health care industries, among others, depend on materials with enhanced hardness. Efforts to discover superhard materials, with a Vickers hardness (H V ) ≥ 40 GPa, have largely relied upon simple design rules and trial-and-error approaches. Accordingly, attempts to mimic the structural and mechanical properties of diamond (H V ≈ 90 -100 GPa), the prototypical superhard material, have manipulated light, main group elements to achieve the desired strong, dense 3D network of short covalent bonds. Cubic boron nitride (c-BN, H V = 55 GPa) and BC 2 N (H V = 65 GPa) are examples of this type. However, the synthesis of diamond and other hard materials in its class is often cost-prohibitive, requiring extreme temperatures and pressures. To combat the exorbitant synthetic costs, researchers have directed focus towards another class of hard materials: transition metal (TM ) borides such as ReB 2 (H V = 45 GPa) and WB 4 (H V = 43 GPa). Integration of the TM s simplifies material synthesis because conventional arc melting processes can yield the desired structures. In these structures, the light boron atoms support the formation of covalent bonding networks, while the heavy TM s are intrinsically incompressible due to their high valence electron density, properties well understood to promote hardness. Incorporating TM s into superhard materials discovery efforts unearths a vast, underexplored compositional space and provides an excellent opportunity to investigate new materials compositions with optimized mechanical properties. Although superhard materials of this type are often impractical as candidates for industrial use as the TM s employed can be expensive and scarce, sustainability concerns can be addressed through elemental substitution of the expensive, rare-earth TM s with more earth-abundant metals. 2 TM substitution can also modify valence electron concentration (VEC ) and atomic size effects to tailor mechanical properties. For instance, substitution of W with Ti, Zr, Hf, Mo, Ta, Mn, or Cr improves hardness of WB 4 -based solid solutions from H V = 43 GPa to greater than H V = 50 GPa. Similarly, Mo 0.9 W 1.1 BC demonstrates improved hardness compared to its isostructural parent composition, Mo 2 BC, with H V = 26.5 GPa and H V = 42.1 GPa, respectively, at 0.49 N indentation load. The Mo 2-x W x BC solid solution system is of particular interest because it provides the unique opportunity to optimize hardness while maintaining moderate ductility, contradictory extrinsic properties often challenging to predict and control. Although hardness and ductility are difficult to predict directly, bulk modulus (K) and shear modulus (G) are intrinsic material properties that correlate to ductility, brittility, and hardness, allowing for indirect estimations. Pugh's ratio, G/K, estimates ductility in inorganic compounds, as compounds with G/K < 0.57 are considered ductile and those ≥ 0.57 brittle. The elastic moduli of hard, isostructural MoWBC and Mo 2 BC have been investigated using density functional perturbation theory, finding G/K = 0.579 and 0.576, respectively. Thus, it is suggested that ductility is preserved in the Mo 2-x W x BC system regardless of the TM ratio, even though elemental substitution of Mo with W results in increased hardness. A separate ab initio study of Mo 2 BC also suggests moderate ductility due to the presence of both the metallic interlayer bonding and stiff carbide and boride layers. The ability to maintain ductility in high-hardness materials is an intriguing concept as conventional superhard materials are typically brittle (i.e. diamond G/K = 1.21). Experimental determination of trends in K within Mo 2-x W x BC solid solutions is a pragmatic step towards informing computational efforts and developing an improved understanding of the balance between elastic moduli, hardness, and ductility in TM -based hard, incompressible materials. Bulk compressibility can be determined through in situ hydrostatic compression experiments in the diamond anvil cell (DAC). In this process, synchrotron diffraction patterns are incrementally collected upon compression, monitoring changes in lattice parameters and fitting an equation-of-state (EOS) to the resultant relative cell volume versus pressure curves. DAC experiments on the tungsten-rich composition, Mo 0.9 W 1.1 BC, revealed its ultraincompressibility with K = 373 GPa. This work investigates trends in hydrostatic compressibility of other Mo 2-x W x BC solid solutions, where x = 0, 0.5, and 0.75, to eluci-date correlations among composition manipulation and physical and mechanical properties of TM -based, hard, incompressible materials. Results will inform future materials design efforts in the quest to optimize mechanical properties while considering synthetic accessibility and material sustainability, providing significant opportunity for applications requiring wear-resistant, superhard materials.