Abstract: A silylation agent for use in gas chromatography is characterized in that it contains fluorine bound to carbon, with the fluorine present in an atomic ratio of fluorine to silicon greater than 4:1. Especially suitable are silyl compounds of the general formula: ##EQU1## in which R' is fluoro-alkyl, perfluoro-alkyl, fluoro-cyclo-alkyl, perfluoro-cyclo-alkyl, fluoro-aryl, or perfluoro-aryl, and R" is hydrogen or alkyl or cyclo-alkyl or their partially or wholly fluorinated derivatives, and the atomic ratio of fluorine to silicon is greater than 4:1. The alkyl groups may contain 1 to 5 carbon atoms.