A semibatch bitumen emulsification process to generate droplet size in the submicrometer range was developed in a coaxial mixer using a high-internal-phase ratio (HIPR) step followed by dilution to the final concentration of the dispersed phase. An HIPR emulsion is characterized by nonspherical droplets shape separated by thin films due to low ratio between the continuous- and dispersed-phase concentrations. The same mixing system was used to prepare the HIPR intermediate and the eventual emulsion, based on a pitched blade turbine on the high-speed shaft and two different types of close-clearance impellers on the low-speed shaft. It was found that the most critical process stage to achieve the mean droplet size was the preparation of the HIPR emulsion. The dispersed-phase and surfactant concentrations, the close-clearance impeller rotational speed, and agitation time were identified as key process parameters to control the droplet size. These parameters were combined to provide a prediction model of the average droplet size with two dimensionless parameters, namely, the initial surfactant concentration in the continuous phase and the bitumen weighted deformation.