Layer inversion in gas-liquid-solid fluidized beds with binary solids was examined in a semi-acrylic column of 1.8 m height and 0.21 m inner diameter. Binary solid combinations were composed of polymer beads (PB, d(p)=3.3 mm, rho(s)=1,280 kg/m(3)) and glass beads (GB, d(p)=0.385 mm, rho(s)=2500 kg/m(3)). Five volumetric ratios (PB:GB =0.67:0.33, 0.6:0.4, 0.5:0.5, 0.4:0.6, 0.33:0.67) were used with superficial liquid velocities of 21.6-34.2 mm/s, and superficial gas velocities of 0 to 16.6 mm/s. Bubble size, bed height, and superficial liquid velocity at the layer inversion point vary with the binary composition. Upon gas introduction into a liquid-solid fluidization column, the superficial liquid layer-inversion velocity increased if it occurred when there was initial bed contraction. On the other hand, the liquid layer inversion velocity decreased when there was initial bed expansion. For all binary solid combinations showing initial bed expansion, liquid layer-inversion velocity decreased with increasing superficial gas velocity in three-phase fluidization, consistent with the gas-perturbed liquid model. (C) 2015 Elsevier Ltd. All rights reserved.