In the present investigation minimum fluidization velocity, U-mf, in a two-phase inverse fluidized bed reactor is determined using low-density polyethylene and polypropylene particles of different diameters (4,6 and 8 mm) by measuring pressure drop. In a glycerol system Umf decreased gradually with increase in viscosity up to a value of 6.11 mPa s (60%) and on further increase there was a slight increase in U-mf. In the case of the glycerol system the Umf was found to be higher when compared to water. In the non-Newtonian system (carboxymethylcellulose), U-mf decreased with increase in concentration in the range of the present study. The U-mf was found to be lower when compared to water as liquid phase. The modified gas-perturbed liquid model was used to predict the minimum fluidization liquid velocity (U-lmf) for Newtonian and non-Newtonian systems. (c) 2006 Society of Chemical Industry.