Fundamental characteristics of liquid-solid fluidization at high pressures (np to 21 MPa) and moderate temperatures (up to 89 degrees C) are investigated Properties under study include minimum fluidization velocity (u(mf)), bed voidage at minimum fluidization, and bed expansion. Devices for in-situ measurements of physical properties of the liquid in the bed are developed and measurements are carried out to quantify tile pressure and temperature effects on the fluidization behavior. The results indicate that under high pressures and moderate temperatures, liquid-solid fluidization behavior is affected significantly by the variation of liquid density and viscosity with pressure. As the pressure Increases, the liquid viscosity and density increase, yielding an increased drag force and buoyancy force on the particles, and hence a decreasing u(mf) and an increasing bed expansion for a given liquid flow rare. Ail increase in temperature has an opposite effect on the physical properties of the liquid, increasing u(mf) and decreasing the bed expansion for a given liquid flow rate. Various correlations proposed in the literature for u(mf) and bed expansion, including those by Richardson and Zaki (1954) and Chitester et al. (1984), are applicable to high-pressure and high-temperature conditions when proper account is made of the liquid physical properties under bed operating conditions.