AbstractSpinodal decomposition has been investigated in Cu–30Ni–2·5Cr and Cu–45Ni–10Cr (all wt-%) alloys, which lie at the Cu rich end and at the centre of the miscibility gap, respectively. Specimens were fast quenched from 950°C for the low Cr alloy and 1050°C for the high Cr alloy, and aged in the temperature range 300–800°C. The progress of spinodal decomposition was followed via hardness measurements, X-ray diffraction, and scanning and transmission electron microscopy. Modulation wavelengths were measured from both X-ray diffraction patterns and electron micrographs. It was found that during the initial stages of aging the modulation wavelength remained constant, whereas hardness increased continuously. After a certain aging time, the hardness remained constant at the peak value, and the modulation wavelength increased continuously. The results are consistent with current theories of spinodal decomposition and hardening.MST/1512