In the control of pests in gas-tight spaces using gaseous phosphine, concentrated phosphine gas is passed through a phosphine-permeable dividing wall and then immediately combined with air to give phosphine-air mixtures with concentrations below self-ignition limits. Control of pests in gas-tight spaces comprises: (a) providing a source of phosphine (PH3) in concentrated form; (b) conducting the concentrated gaseous phosgene from the source into an air stream through a PH3-permeable dividing wall such that the introduced PH3 is immediately diluted at its point of introduction to below the concentration limits for self-ignition of PH3-air mixtures; and (c) introducing the resulting PH3-air mixture into the space to be fumigated. An Independent claim is also included for an apparatus for performing the method comprising a tube fitted in its central region with at least one gas inlet manifold having a porous or PH3-permeable dividing wall with a flat outlet area; the tube can be combined with inlet and outlet pipes for a pressurized air stream; and the manifold can be combined with a concentrated PH3 gas source.