The formation of fines in a fluidized catalytic cracker unit (FCCU) due to catalyst attrition and fracture is a major source of catalyst loss. In addition to standard attrition tests described in the literature, the possibility that thermal conditions could lead to catalyst fracture and fines production has been explored. Samples of fresh and used (equilibrium or e-cat) type catalysts were heated up to 600degrees C and mixed with cold samples to determine the impact of thermal shock on particle stability. It was found that significant fracture occurs under these conditions, leading to loss of larger catalyst particles in the bed and significant gain in the amount of. ne particles. Agglomeration of particles was also evident, in some cases leading to an increase in the quantity of larger particles appearing to be present in the catalyst sample.