The influence of hydrodynamics on the degree of CaCO3 scale suppression by anti-scalants was studied by comparing scale formation in the presence of inhibitors in two different flow configurations. One system consisted of a supersaturated solution flowing as a freely falling film outside a vertical stainless steel tube. In the other system, the supersaturated solution was in full flow through a stainless steel pipe. Tests characterizing scale formation and scale inhibition were performed with the two systems operated under comparable conditions with respect to water chemistry, wall shear stress and temperature. The experimental data showed that suppression of scale deposition by an anti-sealant in the falling film system was considerably more effective than that achieved in the pipe flow system. This rather unexpected effect is rationalized by analysis of the fundamental difference in the mechanisms of scale formation and scale inhibition in the two systems. Results of this study highlight the possibility that system hydrodynamics could be a parameter of no lesser importance than chemical factors in scale suppression by additive treatment.