The influence of ball-mill grinding process parameters and polymer properties on the mechanochemical degradation of amorphous polymers was explored. For process parameters, the grinding frequency was found to have the greatest impact on the degradation rates of polystyrene (PS), with ca. 17 times difference in rate constants between the lowest and the highest frequencies studied. For polymer properties, molecular weight and molecular weight dispersity were shown to both influence degradation rates but in an indirect manner. Linear relationships were found between degradation rate constants and the initial glass transition temperature (Tg) for PS and poly(methyl methacrylate) samples, suggesting that the Tg was a better predictor of the degradation rate than molecular weight (above a limiting molecular weight value). Ultrasonication experiments also further highlighted that polymer properties can have a disparate impact on mechanochemical reactivity depending on the method used to apply mechanical forces to polymers.