The magnetic age hardening produced by decomposition of a &bgr;‐CoAl(B2) alloy has been studied by correlating the microstructure with the changes in magnetic properties during aging. A model based on coherent rotation has been developed to describe quantitatively the intrinsic coercivity of these fine‐particle ferromagnets as well as the temperature dependence of the coercivity over the temperature range from 77 to 757 K. The model incorporates both magnetocrystalline anisotropy constantsK1andK2in addition to the shape anisotropy of the elongated single‐domain particles of metastable HCP cobalt which precipitate from solid solution during heat treatment. The magnetic properties of the particles are shown to depend critically on the crystallography of the solid‐state transformation and particle morphology which were determined by transmission electron microscopy and diffraction.