Unitized　regenerative　fuel　cells　are　unique　devices　that　combine　the　functions　of　fuel　cells　and　water　electrolysis　in　one　system.　The　internal　mass-transport　mechanisms　present　in　a　unitized　regenerative　fuel　cell　are　closely　related　to　the　mode-switching　method　of　the　cell.　A　two-dimensional,　single-phase,　isothermal,　multicomponent,　and　transient　model　was　developed　to　investigate　the　characteristics　of　mass　transfer　coupled　with　electrochemical　reaction　in　a　unitized　regenerative　fuel　cell　under　interval　and　continuous　mode-switching　methods.　Results　indicate　that　the　average　gas　mass　fractions　in　the　gas-flow　channel,　gas　diffusion　layer,　and　catalyst　layer　are　the　same　under　the　two　different　mode-switching　methods.　Gas　mass　fractions　in　different　layers　exhibit　the　same　variation　trend　and　gradient　during　the　first　and　second　cycles　of　interval　and　continuous　switching.　Under　the　two　different　mode-switching　methods,　the　gradient　of　the　gas　mass　fraction　in　fuel　cell　mode　is　larger　than　that　in　water　electrolysis　mode.　The　transient　response　of　these　layers　under　two　different　mode-switching　methods　is　delayed　by　approximately　0.2s　compared　with　that　of　the　operating　voltage.