The thermal decomposition of tert-butylbenzene (TBB) in supercritical water (SCW) was investigated. From experimental data of the overall rate and formation of 30 products (more than 50 experiments; T=500-540 degrees C, p=5-25 MPa, five different environments including SCW) and assuming a free radical mechanism, a high pressure reaction model was developed on the basis of a chemical mechanism. The model, which ultimately consists of 171 elementary reactions (ER), and the chemical mechanism, which identifies the reaction paths for the formation of the main products, were evaluated in an interactive process. For the calculation of the entire set of 342 coefficients of the model, an optimisation method was applied to solve the ＇inverse problem＇, using: (1) as initial values all available kinetic data at normal pressure; (2) best estimates for the remaining coefficients, considering limits for the different types of ER; (3) ＇punishment functions＇, which force the parameters to vary only within certain limits. Model simulations show reasonable agreement with experimental data. The main difference of the reaction in SCW and at low pressure in an inert environment is the strong inhibition of the overall reaction by a factor of 1000. Simulations indicate that mainly radical decomposition reactions are responsible for this effect. It is assumed that a cage effect of water molecules reduces the reactivity of these species. Also, the pressure dependence of the different types of ER is discussed. The product spectra of the reaction in SCW has a greater variety than at 0.1 MPa and below. Simulations show that this is caused by a promotion of substitution reactions and the suppression of decomposition reactions at SCW conditions. Thus, the difference of the decomposition of TBB in SCW regarding the product spectra to the reaction at normal conditions is mainly a pressure effect. These results seem to be of general significance for free radical reactions. They are in agreement with previous studies of the pyrolysis of ethyl-benzene in SCW as well as in inert conditions at atmospheric pressure.