This paperwill try to explainwhy a simple PREN calculationcan not be used to select Duplex Stainless Steels for field applications. Most commercially available materials have to be heat treated(annealed followed byoil or waterquenched). The effect of the heat treatment on the microstructure ferrite to austenite ratio and even the size of the inclusions will be discussed in terms of the corrosion resistance. The discussion will be centered around the experience with commercially available 22 Cr DSS and 25Cr DSS and the influence of the grain size ferrite to austenite ratio and their effect on the corrosion resistance of these materials in different environments (particularly in Cl containing solutions).