A　two-dimensional,　single-phase,　isothermal,　multicomponent,　transient　model　is　built　to　investigate　the　transport　phenomena　in　unitized　regenerative　fuel　cells　(URFCs)　under　the　condition　of　switching　from　the　fuel　cell　(FC)　mode　to　the　water　electrolysis　(WE)　mode.　The　model　is　coupled　with　an　electrochemical　reaction.　The　proton　exchange　membrane　(PEM)　is　selected　as　the　solid　electrolyte　of　the　URFC.　The　work　is　motivated　by　the　need　to　elucidate　the　complex　mass　transfer　and　electrochemical　process　under　operation　mode　switching　in　order　to　improve　the　performance　of　PEM　URFC.　A　set　of　governing　equations,　including　conservation　of　mass,　momentum,　species,　and　charge,　are　considered.　These　equations　are　solved　by　the　finite　element　method.　The　simulation　results　indicate　the　distributions　of　hydrogen,　oxygen,　water　mass　fraction,　and　electrolyte　potential　response　to　the　transient　phenomena　via　saltation　under　operation　mode　switching.　The　hydrogen　mass　fraction　gradients　are　smaller　than　the　oxygen　mass　fraction　gradients.　The　average　mass　fractions　of　the　reactants　(oxygen　and　hydrogen)　and　product　(water)　exhibit　evident　differences　between　each　layer　in　the　steady　state　of　the　FC　mode.　By　contrast,　the　average　mass　fractions　of　the　reactant　(water)　and　products　(oxygen　and　hydrogen)　exhibit　only　slight　differences　between　each　layer　in　the　steady　state　of　the　WE　mode.　Under　either　the　FC　mode　or　the　WE　mode,　the　duration　of　the　transient　state　is　only　approximately　0.2　s.