The absorption spectra, the luminescence properties (at 293 and 77 K), and the electrochemical behavior of six dinuclear heterometallic compounds have been investigated. The compounds are made of Ru(tpy)(2)(2+)- and Os-(tpy)(2)(2+)-type components (tpy = 2,2’:6’,2"-terpyridine, which in some cases carries p-tolyl (Meph) or methylsulphone (MeO(2)S) substituents in the 4’ position), connected by n phenylene (ph) spacers (n = 0, 1, and 2). In the resulting rigid rod-like structures of general formula (X(l)tpy)Ru(tpy(ph)(n)(tpy)Os(tpyX(2))(4+) the metal-to-metal distance varies from 11 to 20 Angstrom. The absorption spectra of the two components are slightly perturbed in the dinuclear compounds, and metal-metal and ligand-ligand interactions are evidenced by the trends of the oxidation and reduction potentials. The luminescence of the Ru-based unit is quenched by the connected Os-based unit with practically unitary efficiency, regardless of the number of interposed phenylene spacers. Quenching is accompanied by quantitative sensitization of the Os-based luminescence. The rate of energy transfer at 293 K is larger than 10(10) s(-1) in all cases. The Forster (Coulombic) mechanism does not satisfactorily account for such a fast rate, particularly for the species with n = 2. It is concluded that the observed energy-transfer processes take place most likely via a Dexter (electron exchange) mechanism. This is consistent with the strong electronic coupling of the Ru-based and Os-based units in the compound with n = 0, and with the relatively small insulating effect expected for the phenylene spacers.