Diphenylcarbenes possessing t-butyl nitroxide (2) and nitronyl nitroxide (3) radicals, potential new spin/magnetic coupling units for extended spin systems, were designed, synthesized, and characterized by ESR spectroscopy. The ESR spectrum of 2 indicated the existence of different conformers, while 3 had only one conformer. These carbenes had quartet ground states determined from the fine structures and the temperature dependence of the signals.