AbstractIn the present investigation minimum fluidization velocity, Umf, 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 Umf. In the case of the glycerol system the Umf was found to be higher when compared to water. In the non‐Newtonian system (carboxymethylcellulose), Umf decreased with increase in concentration in the range of the present study. The Umf 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 (Ulmf) for Newtonian and non‐Newtonian systems. Copyright © 2006 Society of Chemical Industry