The influence of the continuous phase viscosity (mu (c)) on the diameter of styrene-divinylbenzene polymer beads was studied over a large range of viscosities at constant dissipated power. This study was based on the inertial breakup and viscous shear breakup theories for a stirred dispersion. These two theoretical models were compared with an experiment for the two highly viscous agents sucrose and acacia gum [dispersed and continuous phase viscosities (mu (d), mu (c)) = 10(-3) < (d)/mu (c) <1]. We found that the maximum diameters of the polymer beads could not be described by an inertial breakup. The maximum diameters were in good agreement with a viscous shear breakup model for the two viscous agents in turbulent and semilaminar flows. (C) 2000 John Wiley & Sons, Inc.