A new index (SPI) incorporating the theoretical thermodynamic relationships relating the equilibrium and solubility constants to the Gibbs free energy of reactions is obtained for assessing the scaling potential of carbonate system. Also inclusion of activity and specific ion interactive forces extends the applicability of the SPI over a wide salinity ranges and in presence of other cations and anions that is usually encountered in brackish water and sea water reverse osmosis desalination units, in concentrate management, and in differentiating between reverse osmosis and nanofiltration modules that have different passages for monovalent and di-valent ions. Some of the industrially used empirical indices for prediction CaCO3 scaling potential are reviewed and their shortcomings are discussed. The theoretically calculated results for SPI have been compared and validated with experimental values and with other commonly and industrially used indices. The results show that SPI is a better predictive approach. This method could be easily extended to take into account the pressure effects in membrane units. Furthermore, the same principle could be applied and the procedure could easily be extended to other salts. This index is a measure of the scaling potential and provides a conservative measure. Use of Gibbs free energy not only provides a theoretical means for assessing the scaling potential but also will enable future incorporation of the interactive and solvent effects in determining the solubility constants which could become readily possible with the advances in computational chemistry and incorporation of the kinetic and hydrodynamic effects in the future studies from the first principles. (c) 2005 American Institute of Chemical Engineers.