Simultaneous measurements of particle size and velocity with different media are important in the experimental study of multiphase flows. Especially, when one of the dispersed media has a relative refractive index close to unity the optical configuration requires extra consideration in phase-Doppler anemometry (PDA). Such measurements are often encountered when water flows are composed of air bubbles and glass beads with relative refractive indices of and respectively. Large uncertainty in measurements may arise if optical parameters fall into the region of so-called “critical scattering angles.” Furthermore, the scattering intensities from air bubbles and glass beads in water are found to have two-order-of-magnitude differences at an off-axis angle in between 60 to 85 deg, which narrows the measurable dynamic range and causes bias in the size distribution of different dispersed phases. We use a scheme based on the recently developed combined refraction-reflection model and Mie scattering theory to optimize the off-axis angle to minimize the measurement uncertainty. The variation of the critical scattering angles and scattering intensity ratios of media are quantitatively described. The predictions are validated by conducting experiments to measure glass spheres and vapor bubbles in water. © 2001 Society of Photo-Optical Instrumentation Engineers.