Experiments were conducted to investigate the impact of particle density distribution on ebullated bed phase holdups and local fluidization behaviour when operating under high gas holdup conditions. Fresh and spent heavy oil hydroprocessing catalyst having relatively narrower and wider density distributions were compared. A 0.5 wt.% aqueous ethanol solution was used to obtain relatively high gas holdups as observed in many industrial reactors containing liquid mixtures with surface-active compounds. Axial pressure profiles were used to assess the degree of segregation on liquid-solid and gas-liquid solid fluidized beds. While marginal axial holdup variation occurred when operating the liquid-solid fluidized bed, introduction of gas significantly impacted the fluidized bed dynamics by rendering the bed freeboard interface diffuse at low superficial liquid velocity as relatively large bubbles were formed. This was observed visually and experimentally based on the pressure profile curvature. At elevated liquid flow rates, the bed interface became more stable due to smaller bubbles being formed because of the greater shear stress at the gas-liquid distributor. Solid holdup was the most affected by the particle density distribution, where bed expansion/contraction was dependent of the liquid flow rate due to varying particle-bubble dynamics. Such information provided guidance on potential factors that can lead to the loss of bed-freeboard interface in the operation of heavy oil hydroprocessors such as the LC-Finersm. (C) 2017 Elsevier Ltd. All rights reserved.