A couple of commercially available packaging-derived fuels, both obtained as a result of mono-material recycling programs of polyethylene (PE) and polyethylene terephthalate (PET), were fed in a laboratory scale bubbling fluidized-bed reactor, made of quartz. The effect of the main operating variables (bed solids hold-up, inert material size, fluidizing velocity, plastics feed rate) on the agglomeration and the defluidization phenomena was investigated by means of experiments carried out in the bed temperature range of 450-650 degrees C. Different mechanisms of defluidization were identified and characterized. The time at which defluidization occurred was correlated to the ratio between the bed solids hold-up and the polymer feed rate by means of linear relationships. An increase of this ratio, as well as a decrease of the bed temperature, led to a decrease of the defluidization time for both the recycled polymers. No substantial effects were found in the tested range of fluidizing gas velocities. An operating criterion to avoid defluidization was finally proposed.