Usually combination of various salts and other species co-exist in salty waters. However, traditionally the assessment of scaling propensity has been empirical and for a single salt. First the paper reviews the work of the author's research group in the composite scale formation and shows the application of the theoretical Scaling Potential Index (SPI) based on Gibbs Free Energy of reaction which was originally introduced [1] and later developed by the author [2,3] to composite scale formation incorporating previously proposed [4] Pitzer model. Second, it discusses composite calcium carbonate and calcium sulfate formation in batch and RO dynamic system which has shown that co-existence of salts affects thermodynamics, kinetics and scale structure. Third it shows that the ratio of sulfate to carbonate significantly affects composite scale formation and flux decline. Fourth, it uses Sheikholeslami's theoretical SPI (taking into account ion interactive forces, concentration polarization, and osmotic pressure effects) to calculate the onset of scale formation on the membrane surface and compare with experimental results obtained in a reverse osmosis unit; comparison of flux decline and relevant analytical techniques showed that Sheikholeslami's theoretical SPI successfully predicted the onset of scale formation and crystal phases for calcium carbonate and calcium sulfate. (C) 2011 Elsevier B.V. All rights reserved.