The effects of CaSO4 on CaCO3 precipitation were studied in the batch tests at 60, 70 and 80 degreesC in mixtures having calcium carbonate as the dominant salt and at a given total initial calcium concentration of 0.03 M with sulfate concentration ranging from 0 to 0.01 M. Solubility products and rate constants were determined from thermodynamic and kinetic studies and the results indicated that even minute amounts of calcium sulfate affect the thermodynamics, kinetics and the scale structure and no longer the solubility data and rate constants for the pure salt were applicable. Presence of CaSO4 from 0.002 to 0.01 M increased the calcium carbonate solubility product more than an order of magnitude. The effect of salt mixture on the solubility constant of non-dominant salt (calcium sulfate) was reverse as calcium sulfate solubility increased to its pure value with increases in its molar ratio. In addition, the rate equation suggested in the literature (Nancollas and Reddy, J. Colloid Interface Sci. 37 (1971) 824; Reddy and Nancollas, J. Colloid Interface Sci. 36 (1971) 166) for pure salt was not applicable to the experimental data. The general observations indicated that the presence of CaSO4 had weakened the CaCO3 scale which is usually very adherent. The experimental results did take into account the effect of solution ionic strength, however, they suggest that data for pure salt precipitation seem not to be extendable to co-precipitation.