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Mode transitions are significant events in the operation of series-parallel hybrid electric vehicles (SPHEVs) with a clutch serving as the key enabling actuator element. Due to the friction-induced discontinuity of the clutch torque, seamless transition is difficult to achieve. In this paper, a model reference control (MRC) law is proposed to coordinate the motor torque, engine torque, and clutch torque to manage transitions. The control system is overactuated in the sense that three inputs (i.e., three torques) can be manipulated to control the two outputs (angular speeds of the two sides of the clutch). The effects of using different input combinations are analyzed to exploit the overactuation feature of the system, and performance sensitivities to various design factors are studied. The simulation and experimental results from an SPHEV bus demonstrate that the MRC achieves reduced torque interruption, less vehicle jerk, and smaller frictional losses, compared to the conventional operation method.