Experiments are conducted to measure the rise velocity of single bubbles in liquid-solid suspensions at pressures up to 17 MPa and temperatures up to 88 degrees C over the bubble size range from 1 to 20 mm. It is found that the bubble rise velocity decreases with increasing pressure and with decreasing temperature. The decrease of bubble rise velocity is due mainly to the variations of gas density and liquid viscosity with pressure and temperature. The presence of solid particles also reduces the rise velocity; the extent of reduction can be examined in terms of an increase in the apparent suspension viscosity by applying the homogeneous, Newtonian analogy. A mechanistic model is developed which considers a balance of forces acting on a single bubble, including the impact force due to solid particles, as well as buoyancy, gravity and liquid drag forces. Comparisons between the model predictions and the experimental data on the bubble rise velocity in liquid-solid fluidized beds are shown to be satisfactory.