Reduced glass transition temperatures (T(g)) of spin-cast ultrathin polystyrene films with different molecular weights (M(w)) were studied by temperature-dependent spectroscopic-ellipsometry. The film thickness, where T(g) starts declining, was found to depend strongly on the M(w). It turned out that T(g) is well correlated with the overlap concentration, the critical concentration separating the regimes of isolated and overlapped chains in polymer solutions. The correlation well describes the behavior of films with different M(w) that were spin-cast under various conditions. The correlation is indicative of the important role played by unentangled polymer chains in the glass transition of ultrathin polymer films. (C) 2011 Elsevier B.V. All rights reserved.