Grafting of maleic anhydride onto polypropylene was performed in a Haake torque rheometer, according to a central composite experimental design, in which four factors were varied: the maleic anhydride and peroxide concentrations, rotor speed, and reaction time. The current article investigates the effect of rotor speed and reaction time on the level of reacted maleic anhydride, obtained by infrared spectroscopy, and on the extent of degradation, by means of melt-flow index (MFI) measurements. The behavior of the variables studied depends on the levels of maleic anhydride and peroxide concentrations. The increase in rotor speed results in an increase in the percentage of reacted maleic anhydride and a reduction in MFI when the initial maleic anhydride concentration is high, and in a reduction in the percentage of reacted maleic anhydride and increase in MFI when the initial maleic anhydride concentration is low (for the central levels of peroxide concentration and reaction time). The effect of the variable reaction time depends on the remaining parameters.