Synchrotron X-ray scattering measurements were performed in situ during the formation of thin (50–600 Å) overgrowths of otavite-calcite solid-solutions at the (104) cleavage surface of single-crystal calcite. These solid-solutions were precipitated from EDTA-bearing aqueous solutions having varied initial saturation states of otavite and calcite. From repetitive X-ray diffraction scans, the Cd/(Ca + Cd) ratios and the effective thicknesses (average domain size perpendicular to the calcite cleavage surface) of the solid-solutions were determined as a function of time. Additional in-plane X-ray diffraction scans were done to further characterize the relationship between the solid-solutions and the calcite cleavage surface. The solid-solution phase grew epitaxially with a (1014) growth plane oriented parallel to the calcite (104) cleavage surface. The compositions of the solid-solutions evolved with time, while their growth rates (increases in effective thickness) remained fairly constant (10–54 Å/hr). In each experiment, the coverage of the initial surface by the. solid-solution (calculated from the difference between the initial and final Cd concentrations in the aqueous solution) was about 20%. Glancing-incidence X-ray reflectivity scans were also monitored as a function of time. From these scans, we determined that the solid-water interface did not become significantly rougher during the nucleation and growth of the solid-solution phase. These observations indicate that the solid-solution grew by layer spreading and that most growth may have occurred preferentially at macrostep faces produced during cleavage.