A simple double-beam attachment for a single-beam infra-red spectrometer which can be built at little cost is described. No optical modification is needed and a minimum of additional electronic equipment is required for the existing instrument. The method employed is essentially an alternating beam system used in conjunction with a voltage ratio recorder. The beam splitter consists of a demountable twin cell arranged as a rotating sector wheel positioned in front of the entrance slit of the monochromator and is the only addition to the optical system.