For pyrolysis of n-tetradecane (m = 14 in CmH2m+2) at 450-degrees-C under elevated pressures (about 2-9 MPa) for 6-480 min, the major products in the early stage are n-alkanes in the carbon number range of C1-C(m-3) and 1-alkenes in the carbon number range Of C2-C(m-2). There are very little paraffinic products with m - 2 and m - 1 carbon atoms. Formation of the olefinic product with m-1 carbon atom is very limited, but 1-Cm-2H2(m-2) is as abundant as the 1-alkenes in the range of C2-C-11. As compared to the well-known gas-phase pyrolysis, the molar ratios of alkenes to alkanes are much smaller (less-than-or-equal-to 1), except the ratio for the group with m - 2 carbon atoms where the 1-C12H24/n-C12H26 ratio can be as high as 9 in the early stage. This is because 1-C12H24 is produced from beta-scission of a secondary 4-C14H29. radical, whereas n-C12H26 is formed from a primary 1-C14H29. radical, whose population is much less than the secondary radical due to both higher activation energy and the competing isomerization reaction. There appeared a preferential formation Of 1-C6H12 and 1-C5H10 among the 1-alkenes formed after 12 min. This may be attributed to the 1,5-shift and 1,4-shift isomerization of primary radicals formed during tetradecane pyrolysis. The peak of carbon number distribution shifts toward 2 for paraffinic products and toward 3 for olefinic products, and the ratios of alkenes to alkanes decrease with increasing residence time up to 60 min. The general reaction mechanism is characterized by the one-step decomposition of secondary radicals and the 1,5- and 1,4-shift isomerization of primary radicals to secondary radicals. The substrate alkane molecules are the source for hydrogen abstraction in the early stage, but in the later stages the olefinic products also undergo the H-abstraction reactions, which lead to the formation of cyclic alkenes/alkanes and alkylaromatics.