Mechanical spectroscopy is applied to thin polystyrene films of 7.5-730 nm thickness spin coated on a thin silicon reed. Below a thickness of 100 nm, the alpha-relaxation peak (glass transition) broadens considerably and shifts to lower temperatures by a few degrees. These effects are attributed to a different polymer dynamics at the polymer/vacuum and the polymer/silicon interfaces. (C) 2008 Elsevier Ltd. All rights reserved.