The influence of operational parameters and the effect of viscosity on minimum fluidization velocity, U-lmf (minimum fluidization liquid velocity) and U-gmf (minimum fluidization gas velocity) were investigated in a three-phase inverse fluidized bed reactor using low-density polyethylene (LDPE) and polypropylene (PP) particles. The U-lmf showed an increasing trend with an increase in particle diameter. The effect of superficial gas velocity, U-g on U-lmf and the effect of superficial liquid velocity, U-1 on U-gmf for water, glycerol and carboxy methyl cellulose (CMC) as the liquid phase were studied. With an increase in U-k, the U-lmf decreased and with an increase in U-1, the U-lmf also decreased. With an increase in the viscosity (concentration) of glycerol and CMC, U-lmf decreased. Correlations for the prediction of U-lmf in Newtonian and non-Newtonian systems are proposed.