AbstractHomogeneous nucleation of CaSO4·2H2O (gypsum) based on an optical diagnostic technique was studied within a supersaturation range of 1–4 at 25–90°C. The experiments were carried out using an experimental apparatus consisting of a batch crystallizer with the related measurement devices. Signals of scattered and transmitted light coming from a He–Ne laser source were analyzed to measure the induction period (tind), that is, the time delay necessary for homogeneous nucleation to take place. As expected from theory, it was found that tind decreases when either temperature or supersaturation increase; from the dependence of tind on supersaturation, it was possible to distinguish between the mechanisms of homogeneous and heterogeneous nucleation. From the experimental data relative to homogeneous nucleation, the interfacial tension (γs) between CaSO4·2H2O and the surrounding aqueous solution and the activation energy (Eatt) for CaSO4·2H2O crystallization were evaluated. In particular, the dependence of tind on temperature made it possible to evaluate Eatt at 30 kJ/mol and the dependence of tind on supersaturation offered the value of γs to be about 37 mJ/m2, which does not vary with temperature in the interval explored.