This work investigates the hydrothermal conversion of hexadecane, as a heavy hydrocarbon model for heavy oil upgrading, and syngas generation. This experimental analysis was carried out in tubular flow reactor where water and hexadecane react at 525-605 degrees C and 150/220 bar under different residence times. Hydrogen and syngas formation was observed and quantified under the current conditions without significant formation of coke in the reactor. Residual hexadecane was also analysed for its contents of cracking products using GCMS. Investigating the temperature and residence time effects enabled determining the reaction kinetic data from which the activation energy, E-a, was determined to 263 kJ/mol (at 220 bar) and 202 kJ/mol (at 150 bar). The determined kinetic data were compared with previously reported results on high pressure pyrolysis of hexadecane and other hydrocarbons. The effects of increasing the water density by increasing the reactor pressure to (220 bar) was found in particular to enhance heat and mass transfer leading to a higher degree of conversion at lower temperatures, and increasing the ratio of n-alkane to 1-alkenes via in situ hydrogenation. Cracking was found to follow the free radical mechanism under both sub and supercritical conditions, producing nearly equi-molar distribution of n-alkanes and 1-alkenes, C-7-C-13, under lower conversions. Increasing the reaction temperature enhances the formation of 1-alkene via beta-scission, while increasing the pressure increases the formation n-alkanes via H-abstraction. In addition, it is found that the hydrothermal conditions have inhibited the formation of higher molecular weight hydrocarbons, C16+, via addition reactions. Results show the potential for a continuous process for hydrogen generation from heavy hydrocarbons using sub and supercritical water with minimised carbon formation. (C) 2015 Elsevier B.V. All rights reserved.