A variant of the particle image velocimetry (PIV) in combination with the particle tracking velocimetry (PTV) technique is presented for measuring the velocity and concentration profiles of mono- and bidisperse semi-dilute suspensions (up to 4% volume concentration). An object recognition algorithm was applied to locate particles of each species in the plane of the laser light sheet. These data were then processed to obtain particle probability distributions and particle velocities. Unexpected preferential positions were observed in several experiments. Average slip velocities were extracted and converted into drag coefficients. The same were compared with drag coefficients predicted by state of the art correlations, considering sphericity and concentration. Experiments showed evidences of momentum transfer between species. Numerical analysis was performed with the two-fluid method considering monodisperse suspension to explain the cause of preferential positions found in the experiments. (C) 2014 Elsevier Ltd. All rights reserved.