The room-temperature formation of bismuth oxycarbonate (Bi2O2CO3) from Bi2O3 in sodium carbonate buffer was investigated with in situ powder X-ray diffraction (PXRD) in combination with electron microscopy and vibrational spectroscopy. Time-resolved PXRD measurements indicate a pronounced and rather complex pH dependence of the reaction mechanism. Bi2O2CO3 formation proceeds within a narrow window between pH 8 and 10 via different mechanisms. Although a zero-dimensional nucleation model prevails around pH 8, higher pH values induce a change toward a diffusion-controlled model, followed by a transition to regular nucleation kinetics. Ex situ synthetic and spectroscopic studies confirm these trends and demonstrate that in situ monitoring affords vital parameter information for the controlled fabrication of Bi2O2CO3 materials. Furthermore, the β → α bismuth oxide transformation temperatures of Bi2O2CO3 precursors obtained from different synthetic routes differ notably (by min 50 °C) from commercially available bismuth oxide. Parameter studies suggest a stabilizing role of surface carbonate ions in the as-synthesized bismuth oxide sources. Our results reveal the crucial role of multiple preparative history parameters, especially of pH value and source materials, for the controlled access to bismuth oxide-based catalysts and related functional compounds.