Homogeneous nucleation of CaSO4. 2H(2)O (gypsum) based on an optical diagnostic technique was studied within a supersaturation range of 1-4 at 25-90 degrees 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 (t(ind)), that is, the time delay necessary for homogenous nucleation to take place. As expected from theory, it was found that t(ind) decreases when either temperature or supersaturation increase; from the dependence of t(ind) on supersaturation, it was possible to distinguish between the mechanism of homogenous and heterogenous nucleation. From the experimental data relative to homogeneous nucleation, the interfacial tension (gamma(s)) between CaSO4. 2H(2)O and the surrounding aqueous solution and the activation energy (E-att) for CaSO4. 2H(2)O crystallization were evaluated. In particular, the dependence of t(ind) on temperature made it possible to evaluate E-att at 30 kJ/mol and the dependence of t(ind) on supersaturation offered the value of gamma(s) to be about 37 mJ/m(2), which does snot vary with temperature in the interval explored.