The precipitation of beta-Ru phase upon aging of a solutionized Ni-18 at.% Ru alloy was studied by X-ray diffraction and electron microscopy. It was shown that: the beta phase forms by discontinuous lamellar decomposition from nucleation sites on alpha grain boundaries; the phases exhibit well-defined orientation relationships; and the hardness of the alloy correlates strongly with the extent of decomposition. The implications for the use of heat treatment to control the microstructure and mechanical properties of the alloy are discussed.