AbstractMany factors such as temperature, solvent and water‐dosing rate have impact on erythromycin solventing‐out process. This article studies the water dosing effect on erythromycin solventing‐out process measured by focused beam reflective measurement technology. Our experiments showed that the overall number of nucleated particles went through an explosive increasing after the metastable zone. Followed by a slight decreasing, it reached a relative constancy for a long time. This suggests that earlier crystal nucleation can be induced with higher water dosing rate, nevertheless, it is the same amount of water dosed when the overall crystal number reached its peak during the explosive emergence of crystals. After the overall particle number climaxed, further extension of crystal growth time would not bring any significant increase in crystal size. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.