The phase-Doppler technique first introduced by Durst and Zare [1] is now widely accepted as one of the most useful techniques to study local particle velocity, particle size, and particle concentration in particulate two-phase flows. In principle, the technique is also applicable to measure the refractive index of particles, and various methods have been proposed for such measurements. A brief review of work already carried out Is given, providing not only the physical background of the various methods applied for refractive index measurements, but also suggesting those that are most likely to become generally applicable. Among these is the method based on the extension of the standard-phase-Doppler system by a second set of photodetectors. as proposed by Durst and Naqwi [2] and Naqwi et al. [3]. This technique has been studied in detail, and a summary of theoretical and experimental results is provided in this article. Using these results, it is demonstrated that the technique is able to distinguish among particles with different refractive indices, such as bubbles, glass particles, metal particles, droplets, etc. The emphasis of the present article lies in the determination of the resolution of the technique regarding the instantaneous measurement of the refractive index of droplets composed of mixtures of two liquids. Particles of this kind are produced by collisions between droplets of different liquids. The question arises whether the extended phase-Doppler technique is suitable to detect that a collision has taken place, Based on theoretical and experimental results, the detectability of the refractive index is confirmed, and the application of the extended phase-Doppler system to interacting sprays is outlined.