This paper describes the off line processing of chromatographic data from fatty acid methyl ester mixtures by a large digital computer. The output of the gas chromatograph is recorded and stored on magnetic tape; the tape can then be processed at any convenient time. The system takes the analog signal from the electrometer and converts it to digital form, adds a time base and records the data using an incremental magnetic tape recorder. The data sampling rate can be varied from 0.05 to 5 times per second. The total number of samples per tape is limited only by the length of the tape. A 1200 ft reel can store up to 30 hr of gas chromatographic runs. The tape is processed by a CDC‐6600 computer. The computer locates all peaks, determines the retention time and calculates the mass percentage of each peak. Along with the numerical output, a cathode ray tube plot of the data is also generated by the computer. The program has provisions for baseline corrections and peak overlap. The program also incorporates corrections for detector nonlinearity and variation in detector response to different molecular species. The accuracy of the system is within 5% based on analysis of fatty acid methyl ester samples of known composition. Computation time for a 1200 ft reel of tape is approximately 20 min.