Microstructural changes of Ni 4Mo alloy during isothermal ageing at 800 °C were followed by using electron microscopy. The transition from short-range, ordered α-phase (as quenched) to the long-range, ordered β-phase was found to take place by random nucleation and growth of β-phase domains in the α-matrix. After impingement, β-domains coalesce towards a configuration characterized by perpendicular twin plates which are arranged side by side parallel to {110}-type planes. Each of the perpendicular twin plates consists of two types of anti-parallel, twin-related domains.