The microstructural changes during long-term (up to 10,000 h) spinodal decomposition in a duplex stainless steel, 7MoPLUS, have been characterised using TEM, a.c. magnetic susceptibility, X-ray diffractometry, microhardness measurement and optical microscopy. The microstructural changes and deformation microstructures of ferrite and austenite upon spinodal decomposition are characterised. The use of a.c. magnetic susceptibility to study the progress of spinodal decomposition is discussed. Above 450 °C, recent research by K.L. Weng et al. Mater. Sci. Eng. A 379 (2004) 119 [4] has established that spinodal decomposition leads to crisscrossing of dislocations and the development of mottled contrast in the ferrite phase. The present work has shown that at 350 °C (the low-end of the spinodal range), crisscrossing of dislocations still occurs, but mottled contrast is absent. The G phase tends to be in contact with dislocations and its precipitation lags behind the occurrence of spinodal decomposition. No noticeable microstructural changes could be observed in the austenite phase in the spinodal temperature regime. © 2010 Elsevier B.V. All rights reserved.