Exciplex formation at the organic solid-state interface in an organic electroluminescent device has been studied. A new multilayer device consisting of the electron-transport layer of 1,3,5-tris(4-tert−butylphenyl-1,3,4-oxiadiazolyl)benzene (TPOB) and double hole-transport layers of 4,4′,4′-tris(3-methylphenylphenylamino)triphenylamine (m−MTDATA) andN,N′−bis-(3-methylphenyl)-N,N′−diphenyl-[1,l′-biphenyl]-4,4′-diamine (TPD) sandwiched between the electrodes of an alloy of magnesium and silver and indium tin oxide (ITO) emitted bright bluish-green light resulting from the exciplex formed at the interface between TPOB and TPD. The exciplex formation is evidenced by the measurement of the photoluminescence spectra of spin-coated films of TPOB, TPD, and a mixture of TPOB and TPD and their photoluminescence lifetimes.