This study characterizes the effects of pressure, superficial velocity, and particle size on gas interchange between bubble and emulsion phases, in an oxy-fuel fluidized bed under high temperature. A combination of experiments and computational fluid dynamics (CFD) simulation was used, and the experiments were performed by using a high-speed camera to record the bubble characteristics under different pressures. The gas interchange coefficient was calculated by means of a statistical method at the bubble boundary based on CFD simulations. The results suggest that the gas interchange coefficient decreases along with increasing pressure and increases along with increasing superficial velocity and particle size. A formula that considers the influence of pressure on the gas diffusion coefficient is proposed in this work to calculate the gas interchange coefficient between the bubble and emulsion phases in a pressurized oxy-fuel fluidized bed.