For the last twenty-five years most petroleum geochemists and basin modellers have produced and used models for maturation and hydrocarbon generation reactions in geological basins that do not consider pressure as a primary control. These conclusions are based on extensive laboratory investigations mainly using pyrolysis. Chemical theory, however, indicates that endothermic volume expansion reactions, such as maturation and hydrocarbon generation, are controlled by both the system pressure and temperature, and geochemists and basin modellers may need to reconsider the importance of pressure on maturation and hydrocarbon generation reactions in geological basins. Unusual earlier studies (at least in terms of petroleum geochemical pyrolysis research) used a vessel in which the pressure is entirely derived from liquid water rather than both liquid water and vapour, as in hydrous pyrolysis experimentation. Results from these experiments showed that both total organic carbon (TOC) and hydrogen index (HI) were elevated in the pyrolysed kerogen residue, suggesting that hydrocarbon generation was being retarded by the effect of water pressure. This paper presents the results of an experimental investigation into the effects of water pressure and phase on hydrocarbon generation and expulsion from the Kimmeridge Clay Formation (KCF) in the temperature range 310–350°C and in the pressure range 0–500 bar, and presents quantitative results both for the amounts of gas and bitumen generated and the composition of the generated gas. The experimental results show that the water pressure retards both bitumen and gas generation, with gas generation being retarded more severely than bitumen generation.