The effects of CaSO4 on CaCo3 fouling had been studied in the batch tests at 60, 70 and 80°C. Calcium carbonate was the dominant salt and the total initial concentration of calcium was constant in all the runs. The solubility product and rate constants were determined. The results indicated that co-precipitation even in minute amount affect the thermodynamics, kinetics and the scale structure. Solubility and rate constants for the pure salt precipitation are not applicable to co-precipitation. Presence of CaSO4 increased the solubility constant for the calcium carbonate in co-precipitation; the solubility product increased from 65% to 280% when CaSO4 was added to the solution. The rate equation suggested by Nancollas and Reddy  was not applicable to the experimental data. The addition of CaSO4 slowed down caCO3 precipitation. The general observations indicated that the presence of CaSO4 had weakened the CaCo3 scale that is usually a very adherent scale.