The influence of SO(4)(2-) ions on electrodeposition of two allotropic forms of CaCO(3) was studied in artificial underground Tunisian water. The study was performed using electrochemical, gravimetric, and optical methods. Chronoamperometric measurements confirmed the inhibiting effect of SO(4)(2-) ions, especially for concentrations higher than 2 x 10(-2) mol L(-1). The nucleation-growth process was observed by means of an optical microscope positioned behind a transparent electrode. The crystal growth of calcite and vaterite was recorded optically in situ, and image analysis software was used to characterize the instantaneous nucleation process and the growth rate of the crystals. Increased sulfate concentrations were found to modify the calcite morphology, whereas, the crystalline structure evolution of vaterite was found to be independent of the sulfate ion concentration. The use of coupled techniques allowed quantification of the influence of the SO(4)(2-) ions on the crystallization process of CaCO(3).