A non-intrusive, laser-based diagnostic instrument has been developed for the simultaneous measurement of droplet velocity, size, and refractive index in sprays. This instrument consists of a Rainbow Refractometer which has been fully integrated to a Phase Doppler Particle Analyzer (PDPA). The PDPA provides the size and velocity of the droplets whereas the Rainbow Refractometer provides the droplet refractive index. The feasibility of using this integrated diagnostics for the study of liquid-liquid mixing in sprays has been investigated. Two pressure atomizers simultaneously spraying water (m = 1.333) and ethanol (m = 1.361) were used in this study. The results reveal that the instrument has sufficient resolution to successfully discriminate between water and ethanol droplets in the mixed spray. As such, the integrated diagnostics is suited for non-intrusive, liquid-liquid mixing studies involving liquids of different refractive indices.