Despite of quite a considerable amount of theoretical and experimental studies on the kinetics of ultrasonic decisive conclusions about the scheme of bond scission and degradation kinetics have been reached.In view of the recent advances in the determination of molecular weight distribution (MWD) by the method of sedimentation velocity in a θ-solvent,the present authors attempted to attack this problem by MWD studies during the course of degradation process.A benzene solution of polymethylmethacrylate fraction of M_n =9.4 X 10⁵ was used.Six samples were irradiated in ultrasound of 1 megacycles/sec for various durations.The MWD curves of these samples were determined in acetone-ethanol mixed solvent at θ-temperature.Number average molecular weights were calculated from the MWD data.