Pyrolysis of 1,2,4-trimethylcyclohexane (T124MCH) at 30 and 760 Torr has been investigated by using VUV-synchrotron photoionization molecular-beam mass spectrometry. In general, the ambient pressure leads to peak-shaped profiles of light hydrocarbons, while the monotonically increasing curves are observed for light hydrocarbons and aromatics in pyrolysis of T124MCH at low pressure. A detailed mechanism involving 530 species and 3160 reactions developed by the authors previously was used for the simulation of T124MCH with reasonable predictions. The consumption of T124MCH followed similar patterns at both pressures, namely the H-abstractions on the first, secondary, and tertiary carbon atoms are the main consumption reactions of T124MCH. The isomerization reactions transform the C9H17 radicals to branch-chain hydrocarbons, which are the precursors of light hydrocarbons such as ethane and propene. Sensitivity analysis indicates that the CH3 consumption reactions are the most significant promoting reactions at both pressures. Isomerization reactions of T124MCH tend to play significant inhibiting effect on T124MCH pyrolysis. Compared to T124MCH oxidation, T124MCH pyrolysis tends to produce large quantities of benzene and toluene. These results will improve the understanding of the combustion and soot formation of T124MCH as a potential aviation surrogate fuel. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.