© 2014 Elsevier Ltd. This study investigates the effect of water pressure on hydrocarbon generation and source rock maturation at high maturities for a perhydrous Tertiary Arctic coal, Svalbard. Using a 25. ml Hastalloy vessel, the coal was pyrolysed under low water pressure (230-300. bar) and high water pressure (500, 700 and 900. bar) conditions between 380. °C and 420. °C for 24. h. At 380. °C and 420. °C, gas yields were not affected by pressure up to 700. bar, but were reduced slightly at 900. bar. At 380. °C, the expelled oil yield was highest at 230. bar, but reduced significantly at 900. bar. At 420. °C cracking of expelled oil to gas was retarded at 700 and 900. bar. As well as direct cracking of the coal, the main source of gas generation at high pressure at both 380. °C and 420. °C is from bitumen trapped in the coal, indicating that this is a key mechanism in high pressure geological basins. Vitrinite reflectance (VR) was reduced by 0.16 %Ro at 380. °C and by 0.27 %Ro at 420. °C at 900. bar compared to the low pressure runs, indicating that source rock maturation will be more retarded at higher maturities in high pressure geological basins.