The decline in light oil production has triggered the necessity to develop effective processes to upgrade heavy oil feedstocks. An alternative to traditional heavy oil upgrading processes is the hydrothermal partial oxidation in near-critical or supercritical conditions. Partial oxidation leads to the formation of oxygen functionalities in aromatic rings located in central positions within the molecule. These functionalities destabilize the aromatic ring structure, making it more prone to cracking reactions. In this work, the effect of pressure, the ratio between the initial oxygen loaded into the system with respect to the stoichiometric oxygen needed for total combustion (O/O-stoich ratio) and temperature were studied using phenanthrene as model compound for structures commonly found in asphaltene molecules. The process was studied at a range of pressures between 210 and 275 bar, O/O-stoich ratio from 0.1 to 0.75 and temperatures between 360 and 450 degrees C. Changes in pressure did not affect phenanthrene conversion, but had a great impact on product selectivities and the gas and organic-soluble fraction compositions. Increases in O/O-stoich ratio resulted in higher phenanthrene conversions and greater yields to gas. Selectivity to organic-soluble products increased with O/O-stoich ratio to find a maximum at a ratio around 0.2 to then decrease with further increments due to an increase in gas production. Any increase in temperature resulted in higher phenanthrene conversions and also higher yields to organic-soluble products. Yields to gas and coke remained low at all reaction temperatures. Higher selectivity to organic-soluble products was obtained at 230 bar, 0.2 O/O-stoich ratio and 425 degrees C.