A new reaction mechanism generator RMG is used to automatically construct a pressure dependent kinetic model for the steam cracking of n-hexane. Comparison between simulated and pilot plant data shows that RMG is able to generate detailed reaction networks that accurately predict the conversion and the yields of the major products although none of the kinetic parameters are fit to the experiments. RMG generates reaction networks based on minimal assumptions, making it possible to test commonly used assumptions such as the A-hypothesis and the quasi steady-state approximation (QSSA) for mu-radicals, traditionally used in steam cracking,(1,2) as well as in pyrolysis.(3) The RMG-reaction network for n-hexane confirms that no bimolecular reactions of heavy radical species are important at the examined conditions (COT: 953 K 1090 K; COP: 0.20 MPa -0.24 MPa; < 80% conversion), and that the QSSA for the group of A-radicals leads to negligible errors. RMG also offers the possibility to estimate the error introduced by neglecting the pressure dependence of most of the reactions. In the case studied, this frequently made (but seldom tested) approximation appears to be justified.