Ultrasonic degradation of high molecular weight poly(vinylpyrrolidone) is monitored on-line, by light scattering. Degradations are performed in water/methanol mixtures of various compositions at room temperature. The evolutions of the weight average molecular weight and the radius of gyration are monitored. The dependence of the radius of gyration on the molecular weight is found. The second virial coefficients (A(2)) of polymer samples in each solvent are found using Zimm plots. These mixtures are found to be close to theta solvents for poly(vinylpyrrolidone). Degradation rate constants and limit molecular weights are correlated with the radii of gyration and A(2) coefficients of the polymer in the solvents studied. It is found that degradation is more effective in better solvents.