Abstract: Rotation driving apparatus usable with a rotary evaporator for holding a sample vessel in a water bath. The driving apparatus includes a stator supported by an elevating member mounted alongside the water bath, and a rotor mounted on the inside of the stator. A hollow support shaft is mounted within the rotor, and the sample vessel is detachably connected to one end of the support shaft. A concentrator can be connected to the other end of the support shaft, for fluid communication with the sample vessel through the hollow support shaft. The support shaft is affixed to the rotor, so that the sample vessel and concentrator are rotated as the rotor turns. A slitted disk is supported by the rotor, and a photosensor produces signals in response to relative movement of the slits. These signals are used for controlling the rotation of the driving apparatus.