The flow pattern and residence time distribution (RTD) for an endless belt mass transfer device have been studied theoretically and experimentally. Even within the laminar regime the overall flow pattern shows several interesting features, with a wide range of different RTDs, depending on the relative velocities of fluid and belt. At high relative belt speeds the RTDs for cocurrent and countercurrent how approach the same limiting form and at long residence times the system behaves as a single well mixed vessel. The restrictions imposed by the flow behavior on the practical range of operation as a mass transfer device are briefly considered.