The interactions of hardness ions (i. e., Ca, Mg, Ba) with anionic polymeric scale inhibitors (SI) containing different functional groups has been investigated using a turbidity technique. It has been found that all SI tested form insoluble salts with hardness ions under the experimental conditions used (pH 7.0 to 10.0, 25 to 65 °C, 100 to 1,000 mg/L Ca). The results indicate that compatibility of hardness ions with SI decreases with increasing pH, temperature, and hardness ion concentrations and increases with increasing NaCl concentrations. It has also been found that compatibility of hardness ion with SI can be improved by incorporating bulkier and hydrophobic monomer into SI structure. In addition, the data suggest that SI architecture (i. e., molecular weight, monomer functional group, and nature of polymerization solvents) influences the interactions of hardness ions with SI in aqueous systems.