The addition of alpha-, beta-, and gamma-cyclodextrins to ferric tetrakis(3-sulfonatophenyl)porphine complex, [Fe(III)TPPS4], is characterized by optical and magnetic resonance spectroscopy. The formation of the cyclodextrin complex with the metalloporphyrin inhibits dimerization reactions and increases the electron spin relaxation time of the iron(III) center. The water proton nuclear magnetic spin-lattice relaxation rates are reported for the iron(III) porphyrins as a function of the magnetic field strength. The relaxivities for the (cyclodextrin)-[Fe(III)TPPS4] complexes are higher than those of the cyclodextrin-free [Fe(III)TPPS4] at pH 2 and 7. The observed relaxivities for the (cyclodextrin)-[Fe(III)TPPS4] complexes at pH 7 are lower than the values at low pH, presumably because the formation of hydroxo complexes reduces the number of first coordination sphere protons. The electron spin resonance spectra of the (cyclodextrin)-[(FeF2TPPS4)(5-)] complex were measured at 77 K and at room temperature. The spectrum at 77 K displays a superhyperfine coupling to fluorine of 4.17 mT in the g similar to 2 region. The difluorocomplex produces a water proton relaxation rate of 14.73 mM(-1) s(-1) at a magnetic field strength corresponding to 0.01 MHz H-1 Larmor frequency. The shapes of the magnetic relaxation dispersion profiles are not Lorentzian at pH 7 and are not well described by the standard theories for paramagnetic relaxation; however, these results demonstrate that it is possible to construct iron(III) based relaxation agents that produce efficient relaxation of the water protons.