Emulsification at constant shear rate in concentrated conditions (50% in volume fraction) is investigated experimentally by measuring simultaneously the droplet size and the global shear stress using a specially designed rheo-optical "Steady Light Transport" apparatus. The capillary number is varied by changing the continuous phase viscosity, corresponding to disperse to continuous phase viscosity ratios between 0.02 and 2. We show that when the capillary number is large enough (>0.35), emulsification occurs. At constant shear rate, this time-dependant process can be separated into four steps: (1) flow start-up, (2) premix formation, (3) a progressive reduction in droplet size, associated with an increase in shear stress, (4) changes in droplet size and shear stress stop at a well-defined emulsification time. Step (3), called dynamical emulsification, is fully controlled by the critical capillary number and the mechanism of drop size reduction stops when viscous dissipation dominates the droplet elongation and break-up mechanism. This approach accurately describes both the variation in shear stress with droplet size during Stage (3) and the final state of the emulsion in terms of droplet size and viscosity. (c) 2007 American Institute of Chemical Engineers.