Using first-principles calculations within the density functional theory we studied the electronic structure, elastic properties, and stability of M 23 C 6 carbides, where M are the platinum-metals: Ru, Rh, Pd, Os, Ir, or Pt. The lattice constants, elastic parameters, formation energies, and densities of states of M 23 C 6 were compared with those for mono-carbides MC. We demonstrated that these carbides have the positive formation energies and predicted the mechanically stable phases. We found that M 23 C 6 carbides are energetically more favorable than the corresponding MC carbides due to the stronger M-M interactions in M 23 C 6 .Mechanically stable and unstable M 23 C 6 and MC carbides.One of the central issues for the platinum-metal carbides concerns the interplay between their stoichiometry, structure, stability, and properties. For mono-carbides MC, the competing crystal structures such as rocksalt (B1), cesium chloride (B2), ZB (B3), wurtzite (B4), nickel-arsenide (B8), tungsten carbide (WC) were discussed [4]. Attempts were made to understand the influence of stoichiometry and the M/C ratio on the stability of these carbides. The carbon-rich phases (M/C < 1) such as M 2 C 3 and MC 2 (M ¼ Os, Ir) were simulated theoretically [17,18], but as far as we know, no experimental evidences for such phases are available now.The formation of metal-rich phases (M/C > 1) seems to be more realistic, since the majority of the known monocarbides of 3d-5d metals from Groups IVa-Va have a wide homogeneity range from MC to MC 0.50 owing to carbon vacancies, whereas metal-rich carbides, such as M 23 C 6 , M 7 C 3 , M 3 C 2 , M 2 C, or M 3 C may be formed in the related systems consisting of carbon and 3d metals from Groups VIa-VIIIa [2,19,20]. Recently, the properties of substoichiometric carbides M 4 C 3 , M 2 C, M 3 C, and M 4 C (M ¼ Rh, Ir, Pd, and Pt) were simulated in [13,18,21], where the model structures (Ir 4 Ge 3 , anti-CaF 2 , anti-ReO 3 ,