Photoluminescent and electroluminescent studies of bilayer heterojunctions formed from poly(pyridyl-vinylene-phenylene-vinylene) (PPyVPV) derivs. and poly(vinyl carbazole) (PVK) show an emission peak which cannot be ascribed to either the PPyVPV or PVK layer. This peak results from an exciplex at the bilayer interface as demonstrated through studies of absorption and photoluminescence excitation spectra. The photoluminescence efficiency of the exciplex is greater than 20%. The electroluminescence spectrum from the bilayer devices is entirely due to exciplex emission, with internal efficiencies initially achieved exceeding 0.1%. [on SciFinder(R)]