We present an ESR and DFT study of the interaction of cucurbiturils CB[6], CB[7], and CB[8] with ditert-butyl nitroxide ((CH3)(3)C)(2)NO (DTBN) and with spin adducts of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and 2-methyl-2-nitrosopropane (MNP). The primary goal was to understand the structural parameters that determine the inclusion mechanism in the CBs using DTBN, a nitroxide with great sensitivity to the local environment. In addition, we focused on the interactions with CBs of the spin adducts DMPO/OH and MNP/CH2COOH generated in aqueous CH3COOH. A range of interactions between DTBN and CBs was identified for pH 3.2, 7, and 10. No complexation of DTBN with CB[6] was deduced in this pH range. The interaction between DTBN and CB[7] is evident at all pH values: "in" and "out" nitroxides, with N-14 hyperfine splitting, a(N), values of 15.5 and 17.1 G, respectively, were detected by ESR. Interaction of DTBN with CB[8] was also detected for all pH values, and the only species had a(N) = 16.4 G, a result that can be rationalized by an in nitroxide in a less hydrophobic environment compared to CB[7]. Computational studies indicated that the DTBN complex with CB[7] is thermodynamically favored compared to that in CB[8]; the orientations of the NO group are parallel to the CB[7] plane and perpendicular to the CB[8] plane (pointing toward the annulus). Addition of sodium ions led to the ESR detection of a three component complex between CB[7], DTBN, and the cations; the ternary complex was not detected for CB[8]. The DMPO/OH spin adduct was stabilized in the presence of CB[7], but the effect on a(N) was negligible, indicating that the N-O group is located outside the CB cavity. Computational studies indicated more favorable energetics of complexation for DMPO/OH in CB[7] compared to DTBN. An increase of aN was detected in the presence of CB[7] for the MNP/CH2COOH adduct generated in CH3COOH, a result that was assigned to the generation of the three component radical between the spin adduct, sodium cations, and CB[7].