The decomposition of &ggr;‐Mn&sngbnd;Cu alloys containing more than 60 at.% Mn to the mixed (&agr;+&ggr;) phase has been studied using x‐ray and neutron diffractometry and measurements of electrical resistance and Young's modulus of elasticity. There is strong evidence that manganese‐enriched clusters having the &ggr;‐phase structure are produced prior to the precipitation of &agr;‐Mn. When &ggr;‐phase alloys were aged at 450°C for increasing periods of time, they remained single phase, but developed long‐range antiferromagnetic order associated with a tetragonal (c/a<1) lattice distortion, and with a corresponding rise of the Ne´el temperature to an asymptotic value of ∼470°K for all compositions. The effects of aging at other temperatures, and the behavior of furnace‐cooled alloys are discussed in terms of the pre‐precipitation of manganese‐rich clusters.