Literature data on the rheological behavior of a reaction mixture during the graft copolymerization of acrylic acid onto gelatinized starch are scarce. Yet, such information is important for process design. In this work, continuous torque recording was found to be a suitable method to monitor the apparent viscosity. Reactions were performed in a bench scale reactor at ambient conditions with the monomer to starch ratio as the main variable. Apparent viscosity of the reaction mixture increased rapidly, which could be fitted with good accuracy with a first order exponential growth model. The maximum viscosity of 18 Pas was seen at the intermediate monomer to starch ratio of 1.0, a phenomenon that is in accordance with literature data. Starch thixotropy is influenced by the grafting of poly-acrylic acid, until it has almost disappeared at the highest monomer to starch molar ratio of 2.0. POLYM. ENG. SCI., 57:1285-1292, 2017. (c) 2017 Society of Plastics Engineers