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The early stages of the decomposition process in the ternary Cu‐9 wt.% Ni‐6 wt.% Sn alloy have been studied by transmission electron microscopy. The supersaturated solid solution was found to decompose through the spinodal mechanism into periodic and aligned regions which are Sn‐rich and Sn‐lean. It was also found that in addition to this clustering reaction, an ordering reaction also progressed with the ageing treatment to produce a metastable phase with the DO22 (Al3Ti type) structure. However, on prolonged ageing, a discontinuous reaction started at the grain boundaries and consumed the matrix leaving behind colonies of a pearlitic product composed of the equilibrium α (A1 structure) and the γ (DO3 structure) phases. This paper discusses the mechanism of formation of the DO22 phase in the alloy.