A method was developed that observes polymer particles during a polymerization reaction using optical- and infrared imaging. The yielding images give good insight into catalyst-specific properties as shape replication, distribution of activity, and the activation. The advantage of this method is the possibility to link behavior of individual particles to its own specific properties. The infrared method measures surface temperatures of growing particles. Such data are useful to feed mathematical models describing the growth of single particles. The temperature–time curves showed maximum temperature to be reached after some minutes; a simple model was used to describe this effect qualitatively. The maximum particle temperature rise was 20 K, but depended strongly on reaction rate and particle size. Temperature, prepolymerization method, catalyst recipe, and catalyst activation time were varied for a fourth-generation Ziegler-Natta catalyst in polymerization of propylene and ethylene.