Duplex stainless steels are embrittled on exposure to elevated temperatures because of spinodal decomposition (<550°C) and sigma phase formation (between 600°C and 900°C). The sigma phase has been discovered to undergo a paramagnetic-to-ferromagnetic transition at cryogenic temperatures and its Curie temperature has a good dependence on prior annealing temperature. Additionally, it has been found that the room-temperature a.c. magnetic susceptibility also has a good temperature-dependence when spinodal decomposition occurs. It is viable to use room-temperature a.c. magnetic susceptibility and the cryogenic magnetic transition of the sigma phase for 1. temperature measurement and 2. discrimination between spinodal decomposition and sigma phase formation in duplex stainless steels. © 2011 Springer Science+Business Media, LLC.