The stability, in long-term operation. of fluidized bed reactors utilised to pyrolyse plastics waste can be seriously compromised by particle agglomeration phenomena. These phenomena can cause poor fluidization and eventually lead to defluidization. In order to investigate this phenomenon. a polypropylene waste from a food-packaging industry was utilised in a laboratory-scale bubbling fluidized bed reactor. The effect of the main operating variables (bed hold-up, size of inert material, fluidizing velocity, feed rate of plastic pellets) was investigated by means of experiments carried out under an inert atmosphere. The time at which defluidization occurred was correlated with some process variables by means of linear relationships. A new modelling approach was also proposed in order to predict the risk of defluidization under different operating conditions.