AbstractDuring the thermal polymerization of styrene small quantities of dimeric and trimeric styrenes are formed which affect essential properties of the polymer. The kinetics of the oligomer formation are investigated under industrially important reaction conditions and the concentrations of the intermediates formed during the initiation reaction are determined by UV‐spectroscopy. The results show that no gel effect occurs in the reactions investigated. Using the kinetic fomulae and reaction rate constants obtained it is possible to describe the formation of oligomers and intermediates at temperatures of 137°C and 180°C up to 97% monomer convers