A microstructural investigation on a duplex stainless steel (22Cr-5Ni), has been performed. The time-temperature-precipitation curves (T.T.P.) have been defined showing the proneness of this material to decompose in embrittling and corrosion resistance deteriorating phases. After ageing treatments between 600-1250-degrees-C the original two-phase structure decomposes with the occurrence of secondary phases such as carbides (M23C6), nitrides (Cr2N), sigma, chi and R phases. 750-900-degrees-C is the most critical ageing temperature range for the intergranular precipitation of carbides and sigma phase that are known to enhance the sensitivity to intergranular corrosion of the alloy.