Enthalpies of adduct formation of 2,2,6,6-tetramethylpiperidine-N-oxyl have been investigated with various hydrogen-bonding acids. The enthalpies and frequency shifts are compared with results from previously reported correlations. The nitrogen hyperfine coupling constant is found to increase regularly with the enthalpy of adduct formation. The donor orbital in the molecule is identified and a model is proposed to account for the increase in AN with-ΔH. The nmr isotropic shift of the alcohol proton is measured. The difficulties associated with interpreting this shift are presented.