We found that 1-(2-adamantyl)-3-tert-butylaziridin-2-one (5a) is unstable. It slowly decomposes at room temperature, although detectable by IR spectroscopy (1840 cm−1 band in CCl4). On the other hand, a closely related analogue, 1-(2-adamantyl)-3-(1-adamantyl)aziridin-2-one (5b), is very stable, in concurrence with an earlier report [1]. We fully characterized aziridinone 5b, identified its thermal