The purpose of the work is to present a simple test method for assessment of the susceptibility of thermoplastics to reactions under dynamic conditions. It is based on measurements of torque values with the use of a batch mixer. It is well-known that the torque values are strictly related to polymer melt viscosity. On the other hand, temperature changes in the batch mixer due to friction and thermal effects of chemical reactions are restrictions precluding measurements in isothermal conditions. However, it was established that the temperature spontaneously changing in a chamber of the batch mixer may be seen as the opportunity to evaluate the changes of torque induced by chemical processes. For radical modification of eleven ethylene copolymers, it was shown that results obtained according to the method provide valuable information for insight into changes taking place in the reacting system.