Two phosphorus-containing hydrazines, namely morpholin-4-ylphosphoramidic acid diethyl ester (1a) and (2,2,6,6-tetramethylpiperidin-1-yl)phosphoramidic acid diethyl ester (2a), have been synthesized. The corresponding hydrazyl radicals (1b and 2b) have been obtained, by in situ oxidation, and their properties have been investigated by EPR spectroscopy. The 1b radical shows spectra strongly dependent on temperature due to the inversion of the morpholin ring and to rotation about the N-N bond. Since, in the investigated temperature range, both motions take place in the EPR time scale, a kinetic study of these process could be made by analyzing the spectral line-shape variations. The 2b radical is highly persistent and shows a strong temperature and solvent dependence of the phosphorus splitting. The latter property was usefully exploited to study the guest-host interaction of this radical with cyclodextrins. A method is also proposed for the determination of affinity constants for cyclodextrins of nonparamagnetic compounds.