Recent studies on electrophosphorescent polymeric devices have demonstrated that charge-trapping-induced direct recombination on the phosphorescent dopant is of crucial importance. In this paper, we show that the electrochemical properties of phosphorescent molecules, which reflect their carrier-trapping ability, may be a basic design criterion for the selection of host and device configuration. The systems, consisting of a red phosphorescent [Ru(4,7-Ph-2-phen)(3)](2+) dopant and two blue hosts 2-biphenyl-4-yl-5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazole (PBD) and poly(vinylcarbazole) (PVK), are intensively studied. The triplet energy level of PVK and PBD is higher than that of the [Ru(4,7-Ph-2-phen)(3)](2+), and both hosts show the ability of efficient energy transfer to the dopant, however, efficient electroluminescence (EL) is only obtained in the PVK-host system. The combined studies of photoluminescence (PL), EL, and electrochemistry for doped films demonstrate that [Ru(4,7-Ph-2-phen)(3)](2+), which undergoes a multielectron trapping process as it is used as a dopant in electron-rich (n-type) hosts, for instance, PBD, may induce an inefficient recombination for the resulting emission. Whereas using a hole-rich (p-type) polymer, such as PVK, as a host and inserting both hole-blocking and electron-transfer layers can effectively increase the efficiency of the corresponding devices up to 8.63 Cd A(-1), because of the reduced probability of multielectron trapping at the [Ru(4,7-Ph-2-phen)(3)](2+) sites.