Hypersonic relaxation was studied in amorphous polyisobutylene (PIB), atactic polypropylene (PP), atactic poly(vinyl acetate) (PVA), atactic polystyrene (PS), and bisphenol-A polycarbonate (PC) by Brillouin scattering. The hypersonic loss tan δ was determined as a function of temperature. Only a single loss maximum was observed for the polymers studied in this work. The temperatures of maximum loss are 200°C at 4.95 GHz for PIB, 180°C at 4.11 GHz for PP, 200°C at 4.50 GHz for PVA, 240°C at 5.50 GHz for PS, and 280°C at 5.43 GHz for PC. The present results agree very well with the extrapolated secondary glass-rubber transition lines above the temperature where the primary and secondary lines have merged. The loss process is proposed to be local torsional motions of the polymer backbone which lead to structural relaxation of the fluid.