Rotating packed bed (RPB), a novel multiphase contactor, can greatly intensify multiphase mass transfer processes. However, the real-flow situation in a rotating bed, which leads to the high mass transfer rates is still not clear. The first part of this paper describes phenomena and results observed by a video camera mounted on a RPB. The camera rotates synchronously with the rotor so that in situ signal of liquid flow can be acquired, and continuous and stable live images are obtained. Tn the second part of this paper, two electro-conductivity sensors are mounted on a RPB to pick up signals in situ at different positions during rotating. Real RTD in RPB at different operating conditions are obtained. A mathematical model is developed for simulating the velocity profile, and the film thickness of liquid flow on the packing. The calculated residence time in the packing gives an average relative error of 14% in comparison with the experimental results.