In this work, we present direct evidence for the thermally self-initiated polymerization of styrene in the gas phase. Our approach is based on on-line analysis of the gas phase oligomers by mass-selected ion mobility. The mobility measurements provide structural information on the ionized oligomers based on their collision cross sections (Omega) which depend on the geometric shapes of the ions. Theoretical calculations of possible structural candidates of the oligomer ions are then used to compute angle averaged Omega for comparison with the measured ones. The agreement between the measured and calculated Omega of the candidate structures provides reliable assignments to the structures of the oligomers. Furthermore, collisional-induced dissociations of the mass-selected oligomer ions provide further support for the structures obtained from the mobility measurements. Structural characterization, mechanisms of formation and the observed fragmentation pathway of the growing dimers and trimers in the gas phase are presented and discussed. Our results indicate that the gas phase thermal polymerization of styrene proceeds via essentially the same initiation mechanism (the Mayo mechanism) as in condensed phase polymerization. (C) 2011 Elsevier Ltd. All rights reserved.