Coprecipitation characteristics of CaCO3 and CaSO4 were studied under various operating conditions: The investigations were carried out in temperature-controlled batch tests. Coprecipitation results were compared with our experimental results of a single salt crystallization. It was found that the induction period and kinetics of coprecipitation of these two salts follow that of pure CaCO3. However, thermodynamic concentrations of Ca in coprecipitation followed that of pure CaSO4 at all times; The relationship between the thermodynamic concentrations of Ca for pure CaCO3 and CaSO4 solutions depended on the pn of the CaCO3 solution. CaSO4 precipitated in form of gypsum and had a needle shape structure; CaCO3 had a spiral growth and precipitated in form of calcite. The precipitate structure was affected by the co-existence of salts; the re-precipitation resulted in CaCO3 crystals interwoven by CaSO4 crystals. This tends to result in a co-precipitate that is stronger than pure CaSO4 and weaker than pure CaCO3 precipitate.