The ultrasonic degradation of polyvinyl pyrrolidone solutions was carried out in mixtures water and acetone at 25 degrees C to investigate the effect of solvent composition and solution concentration on the rate of degradation. The obtained limiting molecular weights indicate that the extent of degradation was decreased by increasing acetone volume fraction and solution concentration. The obtained limiting molecular weights were correlated in terms of concentration and acetone volume fraction. The calculated rate constants indicate that the rate of degradation was decreased by increasing acetone volume fraction and solution concentration. The calculated rate constants were correlated in terms of concentration, acetone volume fraction and molecular weight of polymer. This degradation behavior was interpreted in terms of vapor pressure of mixtures water and acetone, viscosity and concentration of polymer solutions. Vapor pressure of mixtures water and acetone were increased by increasing acetone volume fraction, and so the vapor easily entered the cavitation bubbles during their growth. This caused a reduction in collapsing shock because of a cushioning effect; therefore, the rate of degradation decreased. As the solution concentration increased, the viscosity increased and caused a reduction in the cavitation efficiency, and so the rate of degradation is decreased. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 120: 3701-3708, 2011