Ruthenium(II) in combination with monodentate, bidentate, and tridentate ligands has proven to be a useful design for a variety of applications, but the majority of systems are virtually nonluminescent in solution. The goal of this work has been to design luminescent forms with practicable emission quantum yields, and the focus has been on [Ru(X-T)(dmeb)CN](+) systems, where X-T denotes 2,2':6',2 ''-terpyridine bearing substituent X at the 4'-position and dmeb denotes [2,2'-bipyridine]-4,4'-dicarboxylic acid, dimethyl ester. Results show that varying the pi-electron-donating ability of the 4'-X substituent is an effective way to tune the energy and lifetime of the charge-transfer (CT) emission. The lifetime achieved in a room-temperature, fluid solution is as high as 175 ns, depending on the 4'-substituent and the solvent employed because the excited state is very polar. That represents a 20-fold improvement in lifetime relative to that of the prototype, [Ru(trpy)(bpy)CN](+), one of the earliest examples found to be luminescent in a fluid solution. A simple theoretical model proves to be capable of rationalizing all the experimental lifetimes. It suggests that, with the dmeb ligand available to accept the electron, enhancing the donor ability of the 4'-X substituent lowers the energy of the (CT)-C-3 state and reduces the likelihood of thermally activated decay via a higher-energy d-d state. However, direct nonradiative decay to the ground state begins to reduce the excited-state lifetime whenever the emission maximum shifts beyond 750 nm. Within those limits, there is inevitably a maximal attainable lifetime, regardless of the method of tuning.