A fixed-bed reactor is described for carrying out reactions of fluid reaction mixtures in the presence of a particulate heterogeneous catalyst having a structured packing which forms interstices in the reactor interior, in which the quotient of the hydraulic diameter for the fluid flow through the structured packing and the equivalent diameter of the catalyst particles is in the range from 2 to 20, preferably in the range from 5 to 10, to such an extent that the catalyst particles are introduced into the interstices, loosely distributed and discharged under the action of gravity.