A magneto‐optical method has been developed to determine the spin‐lattice relaxation times of paramagnetic ions from the maximum rate at which paramagnetism can follow an ac magnetic field modulating a dc magnetic field of 0–6000 G. The spin‐lattice relaxation time &tgr; of the paramagnetic ion determines the maximum rate at which the Faraday rotation of plane polarized light traversing the material may be modulated at low temperatures. By measuring the phase angle between the light modulated by the Faraday rotation and the perturbing magnetic field for both the pure host material and the host material doped with paramagnetic ions, one is able to determine the relationship between &tgr; and the applied static magnetic field at low temperatures. The apparatus and the method employed for this measurement are described.