Relations were developed to describe mass transport phenomena in Parallel Passage Reactors (PPR), dustproof, low-pressure drop reactors consisting of shallow packed beds of catalyst particles confined between wire gauze screens. In a PPR, gas flows along the beds, and reactants are transferred through the screens to the catalyst particles by diffusion and dispersion. At low gas velocities along the beds, interparticle mass transfer in the beds is dominated by diffusion. At higher gas velocities it is significantly enhanced by dispersion, caused by a small gas flow through the catalyst beds. This flow, parallel to the bulk flow, results partly from the axial pressure gradient across the reactor. Up to a few particle diameters from the wire gauze screens, the gas flow through the beds is higher than would be expected from the pressure gradient. This is caused by the higher bed voidage near the wire gauze screen and by convective transfer of momentum from the gas channels, through the wire gauze screens, into the beds. The developed mass transfer relations were used to assess the feasibility of the PPR as a reactor for catalytic denoxing of industrial flue gases. At gas velocities normally encountered in industrial denoxing, dispersion in the catalyst beds of the PPR greatly enhances its efficiency. If the thickness of the catalyst slabs does not exceed six to ten catalyst particle diameters, the reactor performance is controlled more by intraparticle diffusion than by interparticle mass transfer. This holds true, except for very low gas velocities. If this rule of thumb is obeyed, the PPR is an attractive alternative to the Honeycomb Reactor for full Selective Catalytic Reduction of nitric oxide, especially at low temperatures.