Mode　switching　operations　between　fuel　cell　(FC)　and　water　electrolysis　(WE)　modes　are　indispensable　to　unitized　regenerative　FCs.　Complicated　electrochemical　reactions　and　product　transformations　occur　in　the　cell　during　mode　switching.　Thus,　identifying　dynamic　behaviors　during　this　procedure　can　improve　cell　durability　and　system　design.　In　this　study,　the　dynamic　behaviors　of　cell　voltage　and　electrochemical　reaction　during　the　switch　from　the　FC　mode　to　the　WE　mode　are　experimentally　investigated.　Reactant　switching　time　significantly　affects　the　electrochemical　reactions.　The　water　pumped　into　the　cell　in　the　FC　mode　reduces　the　cell　voltage　to　a　negative　value　and　results　in　a　hydrogen　evolution　reaction　at　the　oxygen　electrode　side.　Before　FC　mode　voltage　rapid　decrease　caused　by　supplied　water,　current　transition　could　efficiently　avoided　the　hydrogen　evolution　reaction　at　oxygen　side.　Ensuring　that　the　moment　water　reaches　the　channels　is　close　to　the　moment　of　current　transition　can　improve　the　stability　of　unitized　regenerative　FCs.　(C)　2018　Hydrogen　Energy　Publications　LLC.　Published　by　Elsevier　Ltd.　All　rights　reserved.