An electrochemical quartz crystal microbalance apparatus was inserted in a channel flow cell in order to allow in-situ electrochemical and gravimetric measurements in well-defined electrolyte tube flowing conditions. An expression of the mass transfer towards the electrode was established where the diffusion-limited current is a function of Re-1/3 in the laminar regime and was verified experimentally using the redox couples Fe[CN](6)(4-)/Fe[CN](6)(3-) and O-2/OH-. This hydrodynamically controlled experimental device was developed to investigate scale deposit processes and to evaluate scale inhibitor efficiency. Experiments were performed on a mineral water at different flow rates; in the laminar regime, an increase in the flow rate accelerates the scaling process. The inhibiting properties of a well-known phosphonate (HEDP) were studied and the efficiency of this anti-scale additive was tested at different concentrations and an optimum concentration could be established (0.7 mg dm(-3)). The effect of inhibitor injection during the scale formation could also be evaluated as well as the influence of flow rate on the inhibiting efficiency. HEDP has shown a strong effect on inhibiting crystal growth and that affected the morphology of CaCO3 crystals as well as the coverage of the metallic surface. (C) 2001 Elsevier Science Ltd. All rights reserved.