This article describes the influence of the polymerization rate on the working domain of a counter-rotating twin-screw extruder used as a polymerization reactor. The rate of polymerization was varied by changing the maleic anhydride (Mah) content in the feed, which consisted of styrene, n-butylmethacrylate, and maleic anhydride. It appeared that the Mah content influenced the operating window of the extruder. Higher Mah percentages resulted in higher throughputs and a decreased dependence on the extruder parameters. On the basis of these results, a new theory concerning reactive extrusion was developed, which resulted in a diagram that is useful in optimizing the extruder, when it is used as polymerization reactor. In addition to the effect of Mah on the process, its effect on product properties was studied.