The objective of this study was to convert heptylbenzene (HPB), the simplest model compound of heavy oils, into low-molecular-weight hydrocarbons by suppressing side reactions that produced high-molecular-weight compounds and char. The dramatic variability in the ion product and the dielectric constant of water under supercritical conditions rendered supercritical water (SCW) an acid or a base catalyst. In this study, water was used without any catalyst. The reaction was carried out in an 8.8-mL batch reactor fabricated from Hastelloy C-276. The ability of SCW to decompose HPB was studied at temperatures ranging from 425 to 475 degrees C and pressures from 30 to 40 MPa. HPB was converted into light hydrocarbons such as ethylbenzene, propylbenzene, and butylbenzene, as well as heavy hydrocarbons and gases under the abovementioned conditions. The major liquids produced were toluene, ethylbenzene, unbranched phenylalkanes, branched phenylalkanes, phenylolefins, benzene, and naphthalene. HPB thermolysis followed first-order kinetics, with the following Arrhenius parameters: activation energy, 26.37 +/- 1.21 kcal/mol; log(A, s(-1)), 10.81 +/- 0.50, where A is the pre-exponential factor. HPB pyrolysis in SCW followed almost identical kinetics, and SCW had negligible influence on the reaction path. A low water partial pressure was favorable for reducing the formation of heavier compounds, although the overall HPB conversion was close to 88% at a temperature of 450 degrees C and a reaction time of 60 min. Char formation was drastically reduced in the SCW environment. These results suggested that water acts as a chemical reagent above its critical point (374 degrees C and 22.1 MPa) and aids HPB decomposition.