Scale formation of calcium carbonate on tube walls is studied in a once-through flow system under isothermal conditions. The effects of supersaturation ratio, flow velocity and liquid temperature on the deposition rates are investigated. For relatively small supersaturation ratios (in the range 3.5 to 8) the deposition rate tends to increase with increasing supersaturation. For higher fluid supersaturations with respect to calcium carbonate, the deposition rates remain roughly constant but they are influenced strongly by the fluid velocity. This dependence of the deposition rate on fluid flow suggests a mass transfer-controlled process.