In this paper the retardant effect of nitrilotrimethylenephosphonic acid (NTMP) on calcium Sulfate dihydrate nucleation kinetics is evaluated. by means of an optical technique able to accurately measure the induction period. A laboratory-scale experimental apparatus is used, in which sodium sulfate and calcium chloride are mixed to obtain a supersaturated calcium sulfate dihydrate solution; all experiments are carried out for various values of NTMP concentrations (0.005-0.05 g/L) in solution, while changing the saturation index in the range of 4.04 - 4.82; the temperature was varied in the interval 15-45 degrees C. Experimentally measured induction times are used to evaluate the interfacial tension (gamma s) between crystals and mother solution and to estimate the activation energy (E-att) for precipitation in the presence of NTMP. The measured values of gamma s and E-att are compared with those found in the absence of any additive and in the presence of citric acid, showing that NTMP is a very strong retardant for gypsum nucleation.