In spite of its hydraulic and morphologic importance, the conditions of occurrence of flow separation at the inner bank of open channel bends are still not known. In a series of 23 experiments in a laboratory open channel bend with flat sand bed, it was investigated how the process of inner-bank flow separation depends on two control parameters: the ratio of flow depth H to minimum radius of curvature R, and the Froude number Fr. The Froude numbers investigated were 0.1, 0.2, 0.3, 0.4 and 0.5 and the H/R values investigated were 0.038, 0.053, 0.077, 0.112 and 0.153. All flow conditions concerned strongly curved subcritical flow. Flow did separate from the inner bank in all experiments, but recirculation zones with reversed velocities did not develop. Surprisingly, the flow separation did not show any dependence on Fr. The point of flow separation moved farther downstream in the bend with increasing H/R.