We have used matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry and micro-Raman spectroscopy to identify a quenching species that is formed during operation of [Ru(bpy)(3)](2+) electroluminescent devices. We identify this performance-degrading product to be the oxobridged dimer [(bpy)(2)(H2O)RuORu(OH2)(bpy)(2)](4+) and show this dimer to be an effective quencher of device luminescence. This work is the first to detect a specific chemical degradation product formed during iTMC OLED operation.