The structures of the title nitroxide , C12H16NO (1), and the alkoxy amine derivative, C18H24N2O2 (2), have been determined by single-crystal X-ray structural analyses. Crystals of (1) are monoclinic, space group Cc with a 11.585(6), b 11.697(10), c 16.753(8)?, ߠ94.33(6)ଠZ 8; R was 0.064 for 1226 independent reflections. The -C(NO)C- group is essentially planar (sum trigonal angles 359.9, 359.9ଠas is the five- membered ring. The N-O distances are 1.249(6), 1.252(8)?. Crystals of (2) are monoclinic, space group P21/c, a 8.295(2), b 13.567(5), c 15.294(3)?, ߠ98.82(2)ଠZ 4; R was 0.051 for 1445 independent reflections. By contrast to (1) the geometry about the nitrogen atom is pyramidal (sum trigonal angles 328੬ and the corresponding five-membered ring has an envelope conformation with a nitrogen displacement of 0.45?. The N-O distance is 1.439(4)?. The high-resolution solid state 13C n.m.r . spectrum of (2) is discussed in relation to the X-ray structure.