Solid state magnetic susceptibility data (2-350 K) are presented for the metal-metal doubly bonded dimers [Ru(OEP)](2) (OEP = octaethylporphyrin), [Ru(OETAP)](2) (OETAP = octaethyltetraazaporphyrin), and [Os(OEP)](2). The data are consistent with strong zero-field splitting of the triplet ground state (D approximate to 240-630 cm(-1)). Variable temperature (200-300 K) H-1 NMR data are presented for [Os(OEP)](2) and [Ru(OETAP)](2) and for two heterodimers, [(OEP)RuRu(OETAP)] and [(OEP)OsRu(OETAP)].