A simple modification is described for converting a Perkin‐Elmer Infra‐Red Spectrophotometer to double beam operation for direct recording of percent transmissionversuswave‐length. A phase discrimination system is used, requiring no changes in the optical system, and no sacrifice of versatility or ease of prism interchange. A complete spectrum over the rocksalt region may be obtained in 25 minutes with good resolution, and with no attention from an operator. Samples as small as 1×6 mm can be studied. The system is readily adaptable to other spectrophotometers, including those in the visual and ultraviolet range, the only limitation being sufficiently rapid response by the detector.