The applicability of several classical size prediction models to predict the Sauter mean diameters d(32) of four volume fractions of toluene-diluted heavy oil in water emulsions was studied. Energy dissipation in a fully baffled cylindrical tank stirred at varied speeds by a Rushton turbine was related to the d32 measured with a Mastersizer 2000 laser light scattering instrument after 75 min of agitation. At low oil fractions of 0.01 droplet breakage behaviour was in accordance with the breakage dominance assumption of Hinze-Kolmogorov (H-K) theory. At intermediate oil fractions of 0.05 and 0.1, the system behaved similarly with lower rates of breakage than at 0.01. At high oil fractions of 0.3, the droplet sizes were the largest. Here, coalescence may have played a larger role due to increased collision frequency, while turbulence dampening caused larger eddies and reduced breakage. The resilience to breakage due to the surface elasticity was assumed to be constant for all oil fractions. The experimental diameters compared with the diameters calculated from several size-predictive models showed unsatisfactory predictions at higher oil fractions of 0.05-0.3. By modification of existing constants and coefficients of a H-K-based model that accounts for breakage and coalescence and by using iterative techniques, specific predictive equations for each mixing speed and a new averaged equation were developed. Good agreement between measured and predicted sizes was achieved. Crown Copyright (c) 2006 Published by Elsevier Ltd. All rights reserved.