During commissioning of YiXing pump turbine (Jiangsu province, China) by – at the time – GE Energy Hydro a number of guide vane vibration issues occurred. An investigation was launched to determine the root causes of these vibration incidents including analysis of site measurement data from the incidents and a Computation Fluid Dynamics (CFD) study. Several interesting hydro-dynamic phenomena were discovered during the course of this investigation, notably circumferentially synchronized and amplified von Karman vortices at the guide vane trailing edges in pump mode, unexpected flow attachment to the guide vane trailing edge pump mode resulting in bi-stable flow conditions and a self-excited torsion mode flutter vibration. The latter two phenomena explain the vibration incidents at site. The CFD study helped in identifying and quantifying the geometric parameters that influence torsion mode flutter and therefore enabled a targeted modification of the guide vane profile that is stable with respect to self-excitation. Between May 2009 and April 2010 the modified guide vanes were – now by Andritz Hydro Ltd. – installed in all 4 units of the YiXing pumped storage plant and proved to be successful in eliminating the vibration problems. Opening and closing sequences of the guide vanes – including pump start from closed guide vanes and transition from pump to synchronous condenser operation – could be implemented as required by the contract.