Break-up processes have been investigated in an SMX-Sulzer static mixer. Oil/water and water/oil dispersions have been studied over a range of Reynolds, Weber and viscosity numbers. A semiempirical model has been developed for the prediction of mean drop size by equating the disruptive force acting upon the drop to the cohesive forces due to surface and viscous resistance to breakage. To establish the model, the liquid-liquid dispersions are assumed to be pseudo-fluids and the energy dissipation rate is calculated from pressure drop measurements expressed through a capillary model. Experimental data are correlated with Reynolds and Weber numbers and are analysed in order to show the influence of surface and viscous resistance on the breakage process.