The degradation of high molecular weight polystyrene under turbulent flow was investigated using a rotating disk apparatus for three different solvent systems at a maximum polymer concentration of 150 ppm by weight. The drag reduction efficiency decreases with time due to the mechanical degradation of the polymer molecules, and the extent of the degradation was found to be a function of the solubility parameter of the solvents. A theoretical model for molecular degradation provides excellent agreement with our experimental data.