The mechanism of calcium sulfate dihydrate scale formation on reverse-osmosis membranes was studied by the introduction of a theory of crystallization. According to this theory, the values of the growth rate constant and the waiting period before nucleation obtained in long-term runs were reasonable in comparison with data obtained from the literature cited. From these experimental results, the theory proved to be applicable to scale formation on reverse-osmosis membranes. For practical application, the theory could be extended to binary system solutions containing sodium chloride, which is the major component in sea and brackish water, with adequate approximations. Using this theory, we can predict membrane characteristics during scale formation.