The thermal cracking of n-butylbenzene was experimentally studied at high pressure (70 MPa), and moderate temperatures (583, 603, 623 K), for conversions of n-butylbenzene ranging between 0.7% and 62%. The pyrolysis was performed in sealed isobaric gold tubes (confined pyrolysis). Three main chemical families were observed: short alkylbenzenes (mostly toluene and ethylbenzene), branched alkylbenzenes (isomers of iso-butylbenzene and iso-heptylbenzene) and short alkanes (from CH4 to C4H10). As minor products, alkenylbenzenes (styrene and butenylbenzene), methylindane and biaromatic structures were also quantified. A detailed kinetic model composed of 3542 free-radical reactions and 383 species (molecules and free-radicals) was written in a systematic manner by taking into account all relevant elementary free-radical reactions. A large number of thermochemical and kinetic parameters were computed by theoretical calculations. A very good agreement between experimental and simulation results is observed for every operating condition and for most major and minor compounds. The apparent kinetic parameters were computed at 623 K, 70 MPa and 30% conversion under the assumption of a first-order global rate law: the apparent activation energy was found equal to 66.6 kcal mol−1 and the frequency factor to 6.3 × 1016 s−1. The extrapolation to low temperature (473 K), which is characteristic of deeply buried oil reservoirs, shows that the stability of n-butylbenzene is about the same as the stability of alkanes, but n-butylbenzene is more stable than n-decylbenzene and less stable than toluene.