We describe a method for automatic computation of "methylene unit" retention-time indices in real time on a computer-controlled gas chromatograph/mass spectrometer system. The calculation is based on the exponential relationship between the methylene unit values of hydrocarbon reference standards and the oven temperature at their elution. Using this mathematical relationship, we calculated calibration factors, which eliminates the need to include a set of methylene unit reference standards with each run and simplifies and speeds identification of unknown peaks in chromatograms. Long-term stability and good precision of the calibration factors were observed during the six-month period of study.