Overall and local hydrodynamics and liquid-solid mass transfer mechanisms were investigated in a laboratory scale packed bed reactor operating in gas-liquid downflow. The mean liquid saturation and the liquid-solid mass transfer rate were determined using classical electrochemical techniques and the local instantaneous hydrodynamics were analyzed using electrochemical shear rate sensors. The experimental results as well as theoretical considerations enabled us to contribute to the elucidation of the gas-liquid flow mechanisms, especially in high-interaction regimes. In pulse flow at low liquid flow rates (L < 10 kg m-2 s-1) the wall is, on average, not entirely wetted, this might explain hot spot occurence in industrial fixed-bed reactors. In dispersed bubble flow and in the liquid rich slugs of pulse flow, the local instantaneous liquid-wall shear rate is characterized by high-amplitude-high-frequency fluctuations. The comparison of the space averaged shear rate measurements with the overall mass transfer rate indicates that the liquid-solid mass transfer mechanism is laminar in nature and may be modelized by a succession of developing laminar boundary layers. An overall mechanical force balance on the liquid shows that the average drag of the liquid by the gas is very small, compared to the total energy dissipated by the gas in the reactor. All the experimental results obtained in this work as well as several literature data can be explained by a flow mechanism in dispersed bubble flow, where the liquid flow is dominated by viscous forces whereas the gas bubbles pass through the packed bed by pressure pulses.