Measurements of the optical birefringence were used to probe the relaxation of rubbed ultrathin polystyrene (PS) films on glass substrates. It was found that the glass transition temperature, Tg, of the films dropped by 15−20 K as the film thickness decreased from 10 μm to 5.8 nm. Experiments on thick films (∼10 μm) revealed that molecules closer to the polymer−air interface relax more quickly than molecules farther from the interface. These results are explained in the context of past experimental studies of thermal properties of similar ultrathin polymer films.