As the use of hydrocyclones for liquid-liquid separation becomes increasingly common, the need for a satisfactory method to assess their efficiency increases. Currently available efficiency theories were developed for liquid-solid separations based on the velocity distributions inside a hydrocyclone. These theories, however; appear less suitable for emulsions where the dispersed phase is slightly lighter than the continuous phase, such as oil/water emulsions. An efficiency computation based on the analysis of the trajectories of the droplets is presented. Trajectories are characterized through a differential equation combining models for the three bulk velocity distributions (axial, radial, and tangential) and the settling velocity defined by Stokes’ law. From the critical trajectory and given operating conditions, a characteristic droplet diameter d(100) can be deduced that corresponds to the smallest droplet with a 100% efficiency. Other efficiencies are obtained by changing the initial condition for the trajectory equation. The efficiency results of different hydrocyclone configurations are compared with experimental results, and residual emulsion distributions are estimated.