Structured packed columns, in which the catalyst panicles are enclosed within wire gauze envelopes ("sandwiches") are promising reactor configurations for reactive distillation and hydroconversions. By allowing preferential channels for the gas and liquid phases, counter-current operation is achieved even for millimeter sized catalyst particles without the problem of flooding. This paper reports the results of a comprehensive experimental study of the hydrodynamics of structured packed columns of 0.1 and 0.24 m diameter. The pressure drop is found to increase sharply when the superficial liquid velocity exceeds a certain threshold value. This threshold corresponds to the situation in which a maximum flow of liquid in the packed channels is realized and the excess liquid flows through the "open" channels. The liquid flow in the open channel causes a sharp rise in the pressure drop. A model is developed to describe the holdup of liquid in the open channels. With increasing liquid flow rate, the pressure drop is found to increase exponentially with the liquid holdup within the open channels. Liquid phase residence time distribution studies lead to the conclusion that there is a good exchange of the liquid phase inside and outside the packed channels. The residence time distribution can be described by an axial dispersion model. Compared with a trickle bed reactor, the results of this study show that a structured packed column has a much larger operating window at a much lower pressure drop.