The　process　during　mode　switching　from　electrolysis　cell　mode　to　fuel　cell　mode　is　unclear.　In　this　work,　dynamic　responses　under　different　mode　switching　strategies　from　electrolysis　cell　mode　to　fuel　cell　mode　are　investigated　using　a　unitized　regenerative　fuel　cell　with　an　oxygen　side　transparent　window,　Effects　of　time　interval　between　reactant　and　current　switching,　gas　flow　rate,　fuel　cell　mode　startup　current　density,　and　gas　purging　time　on　the　cell　voltage　are　studied.　The　experimental　results　indicate　that　pre-reactant　switching　is　an　effective　way　to　consume　the　residual　water　at　the　end　of　an　electrolysis　cell　mode,　and　a　sufficient　time　interval　between　reactant　and　current　transition　can　promote　the　smooth　mode　switching　under　low　current　density.　Oxygen　flow　rate　increase　can　promote　smooth　startup　of　FC　mode　when　the　time　interval　between　reactants　and　current　transitions　is　not　long　enough,　and　hydrogen　flow　rate　has　little　effect　on　the　mode　switching　from　EC　mode　to　FC　mode.　In　addition,　the　method,　combining　electrolysis　reaction　and　gas　purge,　can　efficiently　eliminate　the　residual　water　and　prevent　water-starved　condition　when　FC　mode　starts　up.　(C)　2018　Elsevier　Ltd.　All　rights　reserved.