This study addresses the fouling of heat exchangers by coprecipitating inorganic salts. It investigates the effects of nondominant CaCO3 on thermodynamics and kinetics of CaSO4 precipitation as well as on scale structure. Even though precipitating salts coexist in industrial water systems, because of the complexity of the fouling process, research has primarily been concentrated on a single salt precipitation. In addition, all thermodynamic predictive models and softwares only consider the effect of coexisting salts through the ionic strength by Debye-Huckel or modified forms of that. The results of this investigation indicate that the presence of a coprecipitating salt with a common ion affects the solubility constants of the salts, and therefore application of the solubility data obtained under a single salt precipitation is questionable for coprecipitating salts with common ions. Also, the kinetics of precipitation is affected by the presence of a common ion, and the traditional method of kinetic analysis for precipitation of a single sparingly soluble salt is not extendable to conditions where coprecipitation with a common ion effect is present. A comprehensive method of analysis is proposed for coprecipitation with a common ion. Experimental measurements are required to incorporate the effect of common ions in mixed electrolyte systems until such a time that theoretical relationships address this issue. Further study is underway to cover a wider experimental range.