A Study is made of the mechanism of CaCO 3 scale deposition on heat exchanger surfaces, occurring in the turbulent flow of water containing dissolved scale constituents under non-boiling conditions. A general rate model is presented which takes into account physical and chemical steps involved in scale deposition. Controlling mechanisms are elucidated by examining the dependence of scale growth rate on the pertinent basic parameters: flow velocity, scale surface temperature and water composition. Scale growth-rate varies with (Reynolds number) 0.68and is only slightly dependent on surface temperature. This result signifies that CaCO 3 scale deposition is diffusion controlled within the range of surface temperatures (67 to 85°C) and Reynolds numbers (13,000 to 42,000) covered by this study. The experimental data conform to the proposed model and suggest that in certain cases CO 2 formed during the reaction remains in solution, while in others, it escapes as a gas on the reaction surface.