The optical mobilities of particles in a cross-type particle separation system were measured experimentally. Three particles were chosen to test the effects of optical mobility, namely, polystyrene latex, polymethylmethacrylate, and silica particles. The particles, which had the same optical mobility, showed identical behavior even though their sizes and refractive indices were very different. The optical mobility was validated by measuring the retention distance where each particle was deflected by the radiation force. (c) 2008 American Institute of Physics.