The decay kinetics of polystyrene (PS) gratings are monitored by tracking the intensity of the first-order laser diffraction peak as a function of annealing time. For low-molecular-mass PS (24 kg mol(-1), blue circles), an exponential response suggests that the pattern decay is a surface-tension driven viscous flow. In high-molecular-mass PS (1007 kg mol(-1), green circles) a complicated response includes a rapid elastic recovery, a power-law creep, and a viscouslike flow (see figure).