In electron paramagnetic resonance (EPR) nonlinear phenomena with respect to magnetic-field modulation are often studied by out-of-phase spectra recordings. The existence of a nonzero out-of-phase signal implies that the EPR signal is phase shifted relative to the modulation signal. This phase shift is called a magnetization hysteresis. The hysteresis angle varies during a sweep through the resonance conditions for a free radical. By recording this variation, a magnetization hysteresis (MH) spectrum results. In practice, a MH spectrum is computer calculated from two EPR spectra detected with a 90 degree difference in phase setting. There is no need for a careful null-phase calibration like that in traditional analysis of nonlinearities. The MH spectra calculated from second harmonic EPR spectra of spin labels were highly dependent on the rotational correlation time. The technique can therefore be used to study slow molecular motion. In the present work MH spectra and Hemminga and deJager's magnitude saturation transfer EPR spectra (Hemminga, M. A., and P. A. deJager, 1981, J. Magn. Reson., 43:324-327) have been analyzed to define parameters that can describe variations in the rotational correlation time. A novel modification of the sample holder and temperature regulation equipment is described.