All duplex stainless steel samples used for experimental work must be solution treated before any subsequent tests are performed on them. In this work, the effects of solution treatment parameters (temperature and soaking duration) and plastic deformation on the ageing behaviour and ac magnetic susceptibility of two duplex stainless steels, which differ in composition, morphology and ferrite content, have been investigated. Effects of different pre-treatments on the morphology, composition and stability of the ferrite phase have been studied, too. The use of ac magnetic susceptibility for the measurement of ferrite content is feasible, but the effect of plastic deformation must be properly accounted for, or else the ferrite content may be underestimated. The ac susceptibility is dependent upon the composition of the ferrite phase. Dissolution of the ferrite in the range of 650-800 °C obeys the Johnson-Mehl-Avrami equation. Raising the solution treatment temperature may increase the activation energy for transformation. For samples used in most experimental work, performing a very long solution treatment may not be necessary. © 2006 Elsevier B.V. All rights reserved.