An ultrasonic measurement system was developed for monitoring the polymerization of methyl methacrylate (MMA) in a model batch reactor and a model twin screw extruder. Conversion of MMA polymerized in a home-made reaction chamber was compared with results obtained from a differential scanning calorimeter (DSC). The objective is to correlate ultrasonic velocity to extent of reaction based on the model experiments of poly(methyl methacrylate) in solution with MMA. This primary ultrasonic measurement system was applied to the on-line measurement of the extent of MMA polymerization in a model twin-screw extruder.