Two-dimensional gas chromatography/time-of-flight mass spectrometry (GC x GC/TOFMS) is quite effective to analyze organic compounds in crude oils with high resolution and normal-phase column condition (non-polar/polar) was commonly used. Here, to improve the understanding on this method, based on Chinese oil samples, we analyze hydrocarbon fractions in the oils using GC x GC/TOFMS with reversed-phase column system (polar/non-polar) in addition to the normal-phase column system. Results show that the reversed-phase column system is more effective in separating saturated hydrocarbons than normal-phase column system. It is especially effective in generating two-dimensional spectra, because the compounds have a higher separation among different series (polarities). The reversed-phase column system is also particularly effective in separating weakly polar to non-polar saturated hydrocarbons (e.g., branched alkanes and naphthenes of low-to-medium molecular weight). It is also suitable for the isolation and identification of conventional biomarkers such as isoprenoids, steranes, and terpanes. Overall, in the GC x GC/TOFMS analysis of crude oils, the reversed-phase system has distinct advantages over the normal-phase system and specific compounds were identified in detail. Thus, this method deserves more attention. These results and understandings add new knowledge to oil geochemistry and have general implications. (C) 2015 Elsevier Ltd. All rights reserved.