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, T-g, of the films dropped by 15-20 K. as the film thickness decreased from 10 mu m to 5.8 nm. Experiments on thick films (similar to 10 mu m) 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.