IN order to determine true relative retentions or partition coefficients in gas-liquid chromatography the ‘gas hold-up’1 of the apparatus must be subtracted from all retention data measured from the point of injection. It has been recommended1 that the time or gas volume between the injection point and the emergence of the air peak should be taken as the gas hold-up. Moreover, when using detectors which do not respond to atmospheric gases the use of methane has been suggested2.