A dynamic technique using a rotating field has been developed for studying the uniaxial anisotropy of magnetic thin films. It is shown that reversible time-dependent contributions to the anisotropy can be revealed as peaks in a skew versus temperature curve at constant frequency, or in a skew versus frequency curve at constant temperature. Compared to the use of static isothermal annealing, the dynamic technique offers an improvement in the resolution of the components of the anisotropy spectrum. The method has been applied to 900 Å permalloy films prepared by evaporation onto substrates held at 300°C. Measurements conducted from room temperature to 200°C yielded reproducible results, and revealed one rotatable time-dependent contribution governed by an activation energy of 1.2 eV and a period factor of 10-13 sec. The data conform quite well to the formal theory developed for the case of a contribution possessing a single relaxation time. The rotatable contribution accounts for 11% of the total room-temperature anisotropy, and has an absolute magnitude KR∞ of 0.2×103 erg/cm3. The effect can be accounted for by the ordering of iron atom-vacancy pairs. © 1970 The American Institute of Physics.