The liquid-liquid dispersion of two immiscible viscous fluids was studied in an SMX static mixer in the laminar flow regime. The dispersed and continuous liquid phases were silicone oils and aqueous solutions of high fructose corn syrup, respectively. Several different phase viscosities were used and the effect of interfacial tension was examined by adding different concentrations of sodium dodecyl sulphate in the aqueous phase. Flow rates of the two phases were varied, and the effect of the volume fraction (holdup) of dispersed phase was explored for a few cases. The droplets were photographed with backlighting and the images were analysed using the Hough transform method. The drop size distributions for each set of conditions were typically obtained on the basis of about 200 images. Mean drop size was expressed as the mass mean diameter D-43. It was found that the "tail" at the higher end of the droplet size distribution decreased with increasing superficial velocity and continuous phase viscosities and the same effect was seen in the values of D-43. It was also found that D-43 decreased with lowering of the interfacial tension. The effect of the dispersed phase viscosity was significant only at non-zero surfactant concentrations. The holdup was found to have a minimal effect on D-43. A two-stage breakup model has been developed relating D-43 to the capillary number. The model uses the concept that primary drops are formed near the entry to the mixer, with a size determined by the spacing between the cross bars in the element. They are then further broken up by a hydrodynamic mechanism based on an energy balance between the energy input clue to the viscous flow, and the surface energy increase clue to the breakup of the primary drops. This model provides an approximate basis for correlating the data on D-43. (C) 2013 Elsevier Ltd. All rights reserved