The nucleation of gypsum into a NaCl solution was studied within a supersaturation range of 2-4 at 25-70 degreesC. The experiments were carried out using a lab-scale apparatus consisting of a batch crystallizer with the related measurement devices. Signals of scattereded and transmitted light coming from a He - Ne laser source were analyzed to measure e the induction period that is, the time delay necessary for homogeneous nucleation to take place. Comparison of the experimental results with those previously obtained in the absence of added NaCl in solution showed that NaCl strongly affects gypsum precip itation. They were compared in terms of the activation energy and the interfacial tension between crystals and the surrounding aqueous solution.