A comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometer (GC x GC - TOFMS) was used for analysis of gas composition in a constant volume combustion chamber before auto-ignition. For mass spectrometer, electron ionization and field ionization were used in order to estimate molecular formula and identify chemical compounds. n-C7H16/O2/Ar mixture, C7H8/O2/Ar mixture and 20mol%C7H8+80mol%n-C7H16/O2/Ar mixture were used in this study. The amount of reaction intermediates were higher as the temperature or the equivalent ratio were higher. From the comparison between C7H8/O2/Ar and 20mol%C7H8+80mol%n-C7H16/O2/Ar, it was indicated that the reaction of n-C7H16 accelerated the consumption of C7H8 and benzaldehyde. In addition to that, from detected signals of reaction products from 20mol%C7H8+80mol%n-C7H16/O2/Ar, it was suggested that heptanedion was an important product of low temperature reaction of alkanes.