The characteristics of the flow fields in a rectangular channel equipped with cylindrical ribs were examined through the particle image velocimetry experiments. The mean and turbulence characteristics were presented for three different Reynolds numbers of 2900, 8400, and 15000. In addition, the coherent flow structures were extracted by means of the proper orthogonal decomposition method. The flow field is characterized by a large re-circulation region and a secondary clockwise-rotating corner vortex. The high vorticity, fluctuation velocity and shear stress contours are formed along the free shear layers emanating from the upstream rib. As Reynolds numbers increase, the positive vorticity contours extend downstream and fluctuation velocity and shear stress contours spread out towards the... channel wall. The proper orthogonal decomposition results have shown that a horizontally aligned co-rotating vortex pair and a corner vortex are the dominant flow structures for the highest Reynolds number.