The aim of this paper is to measure the induction period for gypsum nucleation when 2-phosphonobutane-1,2,4-tricarboxylic acid (also PBTC, commercial name Bayhibit) is added to a solution as an additive. A well-assessed laser light scattering technique previously devised for the measurement of induction times has been used. The induction period values measured are then used to estimate the values for the interfacial tension between gypsum crystals and the mother solution for three PBTC concentration levels (0.05, 0.10, and 0.50 g/L) at the temperature levels of T = 15, 25, and 35 degrees C, according to different nucleation mechanisms. Starting from the dependence of induction times on temperature, the activation energy for nucleation is estimated for each tested PBTC concentration, allowing the determination, with an a posteriori analysis, of which is the governing mechanism in gypsum nucleation kinetic, and thus the more reliable interfacial tension values.