The quasielastic light scattering (QELS) of poly(methyl methacrylate) was measured as a function of both temperature and free monomer concentration, which was varied by heating. The motion responsible for the QELS is not thermally activated (i.e., the corresponding relaxation time is independent of temperature), while the QELS intensity is strongly temperature dependent and proportional to the monomer concentration. The QELS is interpreted as due to a rapid localized motion released by thermally activated slow structural relaxation, which can create free volume.