Alloys near the composition PtCo can be quenched to retain a disordered fcc phase. Aging at a suitable temperature produces an ordered tetragonal phase of type AuCuI. The C axis is an easy axis of magnetization with an anisotropy constant of ∼ 107erg cm−3. Aging produces a magnetically hard alloy. In the early stages of aging ordered regions of diameter 50–100A˚ were found with their C axes oriented at random parallel to one or other of the three original disordered cubic ′a′ axes. In the optimum high coercivity state particles of like C axis (diameter ∼200A˚) group together on {110} planes; the c axis being at 45° to the {110} plane. In the overaged state like particles merge to form lamellae of one particular C axis the other orientations being consumed. Approaches to a better understanding of the hardening process are discussed.