Experiments of pressure effects on gas holdup and bubble size in slurry bubble columns at 5.6 MPa and at gas velocities up to 45 cm/s indicate that the gas holdup increases with an increase in pressure, especially at high slurry concentration. At ambient pressure, a higher solids concentration significantly lowers gas holdup over the entire gas-velocity range, while at 5.6 MPa, the effect of solids concentration on gas holdup is relatively small at gas velocities above 25 cm/s. An empirical correlation was developed based on these data and those in the literature to predict gas holdup in bubble and slurry bubble columns over a wide range of operating conditions. An analysis of bubble flow characteristics during dynamic gas disengagement indicates that large bubbles play a key role in determining gas holdup due to the large bubble and wake volumes that induce the acceleration of small bubbles. Direct measurement of bubble size shows that elevated pressures lead to smaller bubble size and narrower bubble-size distributions. Bubble size increases significantly with increasing solids concentration at ambient pressure, while at high pressures this effect is less pronounced. A theoretical analysis of circulation of gas inside the bubble yields an analytical expression for maximum stable bubble size in high-pressure slurry bubble columns. Based on this internal circulation model, the maximum stable bubble size at high pressures is significantly smaller due to the high gas inertia and low gas–liquid surface tension. The smaller bubble size and its reduced bubble rise velocity account for the observed pressure effect on gas holdup.