Homogeneous line widths, characterized by the phase memory time, T., of a nitroxide spin-labeled peptide, were measured over the entire motional range (similar to10(-10)-10(-4) s at the spectrometer resonance frequency of 9.7 GHz) using spin-echo correlated spectroscopy (SECSY) ESR. Phase memory times as short as 20 ns could be determined by using schemes designed to minimize the effects of pulse ring-down. Experimental T-m versus temperature curve (from 192 to 310 K) clearly resolves the T-m minimum, which provides great sensitivity to the details of molecular dynamics.