Expansion of pentane‐swollen polystyrene beads was followed at and above the glass transition by using a very simple optical technique. Measurements allowed a suitable determination of the glass transition temperature and of the viscoelastic characteristics of the pentane‐plasticized polystyrene chains inside the beads, at different pentane contents. This was achieved by combining the well‐known Williams, Landel, and Ferry approach and some calculations derived from a recent micromechanical modeling of the expanded polystyrene microstructure. The results, analyzed in terms of free volume characteristics, revealed the peculiar plasticizing character of pentane, as compared to the usual polymer diluents. A value of 42°C was found as the lowest temperature for which the dimensional variation of the beads could result from glass transition motions. The relevance of these conclusions was discussed, not just in terms of the changes in dimensions of the individual beads at zero expansion time but also in terms of the dimensional evolution observed in the expanded polystyrene structures.