The potential use of monolithic reactors for carrying out gas-phase catalytic reactions Is discussed. A comparison is made between the monolith and the classical fixed bed reactor, from which it is concluded that the monolithic structure offers a convenient way to put a given amount of catalytic material into contact with a flowing reactant avoiding pressure drop and internal mass-transfer limitations. Heat- and mass-transfer effects are discussed, and it is shown that unusually low Sherwood numbers for external mass transfer reported in the literature might result from a residual contribution of heterogeneous reactions, even under well established mass-transfer control.