The formation kinetics of CaCO3/BaCO3 and BaSO4/SrSO4 solid solution systems is studied in this research. A CaCO3-precoated stainless steel plug flow reactor is developed to study the CaCO3/BaCO3 system. Ba2+ is found to partition into Calcite solid at low concentration, but at high concentration, it can ion exchange for the Ca2+ ions or poison the Calcite surface and result in dissolution. Beaker study of BaSO4/SrSO4 shows the presence of Ba2+ can accelerate the precipitation of SrSO4. Also a linear relationship is found between the saturation index of BaSO4 and SrSO4 during the precipitation process. The influence of background electrolyte is also discussed. Future research is needed for investigating the influence of different temperature, ionic strength, Cation and Anion ratio and the existence of other common ions in water to solid solution formation.