Ultrasensitive, thin-film, differential scanning calorimetry is used to determine the glass-transition temperature of 3-400 nm thick, spin-cast films of polystyrene, poly (2-vinyl pyridine), and poly (methyl methacrylate) on a platinum surface. The technique used here is modified to characterize the glass transition over a time scale of seconds rather than milliseconds. No appreciable dependence of the glass-transition temperature on thickness is observed over the entire range used. The results are discussed in comparison with previous calorimetric measurements, characterized by the glass-transition time scale of milliseconds.