Using several complementary microstructural analysis techniques, the precipitation of sigma (a) phase has been studied in detail in a superferritic stainless steel W. Nr. 1.4575 (DIN X 1 CrNiMoNb 28 4 2). Sigma phase precipitation kinetics has been studied at temperatures of 600, 650, 700, 800, 850 and 900degreesC. Volume fraction changes with time, for isothermal treatments, could be described by the Johnson-Mehl-Avrami-Kolmogorov equation. TTP diagrams for the sigma phase have been also determined. Precipitation of the sigma phase conduced to an extremely fragile steel, especially in those situations where grains boundaries were completely covered by sigma phase.