Multiphase fixed-bed reactors have complex hydrodynamic and mass transfer characteristics. The modeling and scale-up are therefore difficult. The present work focuses on the role of mass transfer on the effective reaction rate. The catalytic 1-octene hydrogenation was taken as a model reaction. The reaction rate in the trickle-bed reactor is by a factor of 20 smaller than (theoretically) in the absence of any mass transfer limitations. For high octene concentrations (> 10%), the effective reaction rate is limited by the H-2 consumption, above all by the gas/liquid and liquid/solid mass transfer. For lower octene concentrations the reaction is zero order with respect to H-2 and only depends on the octene consumption, i.e., on the interplay of chemical reaction, US and intraparticle mass transfer of octene.