Induction periods for the nucleation of barium sulfate were : experimentally determined as a function of supersaturation in NaCl solutions from 0 to 6 m at 25 degrees C and in 1.0 m NaCl solutions at temperatures from 25 to 90 degrees C. Employing the classical nucleation theory, the interfacial tension between barite and supersaturated solutions with NaCl was estimated to be about 93 mJ/m(2) (less : than 0.003 m NaCl) and 79 mJ/m(2) (1 m NaCl) at 25 degrees C. It decreases slightly with increasing temperature (to 71 mJ/m(2) in 1 m NaCl solutions at 90 degrees C). It was observed that the induction period decreases with increasing NaCl concentrations to 3 m and then starts to increase slightly at a given temperature and saturation. index. The electrolyte effect on nucleation was proportional to the enhanced solubility of barium sulfate in NaCI solutions. This phenomenon may result from decreased crystal-solution interfacial tensions caused by increased barite solubility in electrolyte : solutions. The increase in induction period at high NaCl concentrations may be due to the substitution of Na+ ions into the lattice : of BaSO4.