Effect of scale on the drop size distribution in a pump-mix mixer has been studied. A network of zones model was developed to predict the drop size distribution at different locations in the mixer. Computational fluid dynamics model was used to obtain the flow patterns in the mixer and to identify zones based on the flow patterns. Population balance equation was solved for all the zones of the mixer. The model was validated with the experimental data over a wide range of parameters as well experimental data from the published literature. The model was further extended for scale-up studies. Two different scale-up criteria were studied. it was observed that equal power consumption per unit mass and geometrical similarity is a better scale-up criterion as compared with equal tip speed criterion for pump-mix mixers. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.