Mixed-metal borides are promising superhard materials (Kaner et al. (2005) [1]). In this Letter, density functional computations have been applied to the structural, electronic and elastic properties of mixed-metal diborides Re0.5Ir0.5B2, Re0.5Tc0.5B2, Os0.5W0.5B2 and Os0.5Ru0.5B2. The elastic moduli decrease from pure metal diboride ReB2 to Re0.5Ir0.5B2 and on the contrary increase from OsB2 to Os0.5W0.5B2 because boron-metal interactions are contaminated by the occupied anti-bonding orbitals. Alloying ReB2 (OsB2) with Tc(Ru) decreases the elastic moduli owing to the relativistic effects. Mixed-metal effects on elastic deformations focus on bonding strengths, which effectively tune the elastic properties of metal diborides. (C) 2010 Elsevier B.V. All rights reserved.