In centrifugal pumps, the interaction between the rotating impeller blades and the stationary diffuser vanes generates specific pressure fluctuation patterns. At off design operation, high incidence and large local pressure gradients can lead to flow separations in rotating or stationary components of centrifugal pumps. The resulting rise of mechanical vibration levels may negatively affect the operational performance and the life span of mechanical components. This paper presents detailed pressure fluctuation measurements performed in a high speed centrifugal diffuser pump stage at full scale in the stationary and in the rotating domain at various locations for different operating conditions. The analysis of pressure fluctuations experienced at the impeller blade trailing edge clearly allowed the detection and exploration of flow separations in the channels of the vanned diffuser. The observed flow separation in the diffuser channels can be stationary or rotating with different rotational speeds and number of stalled channels, depending on the relative flow rate of the pump. The non-uniform pressure distribution at the impeller periphery, which is depending on the flow separation within the diffuser channels, changes the impeller deformation pattern and creates high local stresses in the junction between the impeller blades and the impeller shrouds. The detection of rotation or stationary stall by using either shaft vibration measurements or bearing force measurements has been demonstrated to be a useful mean of avoiding unfavourable pump operation.