The continuous precipitation of calcium carbonate is studied here with on-line particle size distribution evaluation and in-situ video visualization. Morphological characterizations of collected samples using differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and cryo-SEM were also performed. It is found that in a scaled reactor the nucleation is homogeneous and predominantly of vaterite which transforms to calcite within the reactor itself, and subsequently outside the reactor during sample collection and drying. In an unsealed reactor the precipitation is predominantly heterogeneous with both vaterite and calcite being nucleated. The heterogeneously nucleated material is morphologically more perfect and is therefore slower to transform and has a lower growth rate. As a result of the transformation of vaterite, conventional characterizations such as DSC, XRD, and SEM, involving sample collection, filtration, and drying, at room temperature are unreliable. More meaningful determinations are available from cryo-SEM studies of rapidly frozen samples in which the transformation and subsequent nucleation is arrested.