An optical method is presented to measure simultaneously and separately the velocity field of both phases in particle-laden flows. The fluid is seeded with flow markers which are fluorescent at a specific wavelength and thus can be distinguished from the sediment particles by applying an optical filter. The motion of each phase is recorded by two CCD cameras, which are triggered such that a high correlation between subsequent images is guaranteed. The velocity fields are determined by means of least-square matching of a group of particles. The whole set-up was applied to study the sedimentation of particles through a rapidly evolving mixing layer.