Polymer particle agglomeration and melting causing sheet formation in fluidized bed reactors have been reported to occur during olefin polymerization. The cause of this deleterious sheet formation has not yet been established. Infrared and optical imaging were used to study the temporal temperature and size during polymerization of individual commercially active catalyst particles supported on a nylon mesh. Experiments conducted at 2100 kPa confirmed theoretical predictions that the temporal temperature rapidly attains a maximum after the start of the reaction. The magnitude of the temperature rise increases upon an increase of either reactor temperature, initial catalyst particle diameter, catalyst loading, or addition of hydrogen to the feed. The maximum transient temperature rise of single commercial catalyst particle during ethylene polymerization (7.5 degrees C) is lower than that which can lead to local melting and polymer sheet formation. The temperature rise of particle clusters is much higher than that of individual particles, as the cluster acts as a particle with an effective diameter much larger than that of an individual particle. Thus, to circumvent polymer melting it is essential to avoid catalyst particles clustering, which may be caused by electrostatic attraction. The addition of hydrogen to the feed, often used to control polymer properties, significantly increased the initial particle temperature rise (almost double in some cases). (c) 2008 American Institute of Chemical Engineers.