A process for coating internals in a reactor, except for the coating of electrically heatable, at least partly open-cell foams, with a catalytically active material or a precursor thereof is described, in which an aerosol which contains, as a disperse phase, the catalytically active material or the precursor thereof is provided and the aerosol is passed through the reactor at a velocity which is established so that the disperse phase of the aerosol is deposited on the internals in the reactor.