The deleterious constituents sigma, chi, and eutectoid gamma(2) can precipitate during hot forming and welding operations in superduplex stainless steels. In this study the precipitation of these phases in superduplex UNS S32750 was investigated. Two materials with similar composition and quite different grain sizes were used in the study. Specimens were subjected to isothermal treatments in the 800-950 A degrees C temperature range for time periods up to 240 min. Under these conditions, quantifications of deleterious phases by light optical microscopy were carried out in order to model the precipitation kinetics of these deleterious constituents. A sigmoidal equation, similar to the one developed by Johnson-Mehl-Avrami was fitted to all the analyzed temperatures. Precipitation velocity was evaluated so as to compare the kinetics of deleterious phases in the two materials. The activation energies for precipitation were calculated based on Arrhenius equation.