The structural arrangements of the ternary metal borocarbides MB 2 C 4 (M ¼ Mg, Ca; La and Ce) are investigated using densityfunctional theory (DFT) calculations within the generalized gradient approximation (GGA). Results indicate that these compounds adopt a layered structure consisting of graphite-like B 2 C 4 layers alternating with metal sheets. Within the hexagonal layers, the coloring with the -C-C-C-B-C-B-sequence is energetically more stable than that with the -C-C-C-C-B-B-one. The electronic structures of these compounds, mainly determined by the B 2 C 4 sheets, can be rationalized with the simple valence electron distribution M 2+ [B 2 C 4 ] 2À xe À , with the metals essentially acting as two-electron donors with respect to the boron-carbon network, the other x electrons remaining in the relatively narrow d and/or f bands of the metals. Accordingly, MB 2 C 4 are narrow band-gap semiconductors (DEE0.2-0.4 eV) with M ¼ Mg and Ca. On the other hand, with M ¼ La and Ce, the compounds are conducting with a relatively high density of states at the Fermi level predominantly metal in character with substantial B/Cp* antibonding state admixture.