Calcium carbonate scale formation on the external surface of a heated tube was studied in a flow-through setup. This setup was effective in isolating the effect of supersaturation from that of temperature and allowed nondestructive examination of the scale morphology. The initial scale layer consisted of a polymorphic mixture, and the scale thickness increased with time and supersaturation. Varying the heating medium temperature changed the polymorphic distribution from a calcite/aragonite mixture to pure aragonite. Linear scale growth rates were determined for two inlet supersaturations. For the highest level of supersaturation, the growth rate decreased with time as a result of slower growth front kinetics due to the lower skin temperature resulting from increased scale thickness.