This study introduces a novel reactor concept, referred to as the Siphon Reactor, for intensified phase contacting of gas-liquid reactants on heterogeneous catalysts. The reactor comprises a fixed catalyst bed in a siphoned reservoir, which is periodically filled and emptied. This serves to alternate liquid-solid and then gas-liquid mass transfer processes. As the duration of each phase can be manipulated, mass transfer can be deliberately harmonized with the reaction. Residence time experiments demonstrate that, in contrast to periodically operated trickle-bed reactors, the static liquid hold-up is exchanged frequently and uniformly due to the complete homogeneous liquid wetting. A mathematical model describing the siphon hydrodynamics was developed and experimentally validated. The model was extended to account for a heterogeneously catalyzed gas-liquid reaction and capture the influence of siphon operation on conversion and selectivity of a consecutive reaction. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 208-215, 2017