Abstract: Based on principles of physicochemistry, this paper studies fluid pressure and phase states of gas (oil) bearing fluid systems. The fluid pressure consists of hydrostatic pressure and system pressure. The later is a function of fluid properties (composition, volume and temperature) of the fluid system, and it can be quantitatively calculated by relevant components. According to components and saturation degree of gas and oil, the phase states are classified into eight types. Then, major factors (temperature, pressure and composition) influencing the phase transition are discussed by using equation of state for a typical fluid system. This provides a new approach to quantitative study on properties and states of gas (oil) bearing fluid systems.