The crystal and molecular structures of 2-methyl-2-phenylindolin-3-one-1-oxyl and 2-methyl-2-phenyl-1,3-indandione are reported and the geometry of the molecules is discussed in relation to that reported in the literature for analogous systems. The two compounds are isomorphs and consequently it was possible to obtain crystals mixed in any ratio. Crystals of the ketone with 1% of the nitroxide radical have been studied by EPR and ENDOR spectroscopy. The hyperfine tensors of the protons and of the nitrogen nucleus, and thegtensor have been determined. The principal directions of the magnetic interactions have been compared with those obtained from the crystal structure. The presence of two diastereometric conformations of the radical in the mixed crystals is discussed.