Nitroxyl free radical electron spin relaxation times for spin-labeled complexes of low-spin iron(III) porphyrins were measured between 8 and 70 K by two-pulse spin-echo spectroscopy and between 8 and 120 K by saturation recovery. Relaxation times for low-spin Fe-III(TPP)(MeIm)(2) (TPP = tetraphenylporphyrin; MeIm = methylimidazole) were measured between 10 and 28 K by saturation recovery and between 10 and 25 K by electron spin-echo. At low temperature the iron electron spin relaxation rates are slow relative to the electron-electron spin-spin splitting. As temperature is increased, the relaxation rates for the Fe(III) become comparable to and then greater than the spin-spin splitting, which collapses the splitting in the continuous wave EPR spectra and causes an increase and then a decrease in the nitroxyl spin-echo decay rate. Throughout the temperature range examined, interaction with the Fe(III) increases the spin-lattice relaxation rate (1/T-1) for the nitroxyl. The measured relaxation times for the Fe(III) were used to analyze the temperature-dependent changes in the spin-echo decays and in the saturation recovery (T-1) data for the interacting nitroxyl and determine the interspin distance, r. The values of r for the four complexes that were examined were between 10.5 and 15 Angstrom, with good agreement between values obtained by spin-echo and saturation recovery. For each of the compounds the value of r is consistent with other data for that spin-labeled porphyrin. Analysis of the nitroxyl spin-echo and saturation recovery data also provide values of the iron relaxation rates at temperatures where the rates are too fast to measure directly by saturation recovery or electron spin-echo spectroscopy. These data demonstrate the power of time-domain EPR measurements as a probe of distance between a slowly-relaxing spin and a relatively rapidly relaxing metal ion such as low-spin Fe(III).