Laboratory and in-line dielectric spectroscopy studies of molten polymers revealed the ability to make composition measurements of the comonomer concentration along the polymer chains. The dielectric permittivity or capacitance was found to be proportional to the polymer sidechain composition. The length of the polar sidegroup dictated the optimum measurement frequency: i.e., a short sidechain, such as the C-Cl bond in chlorosulfonated polyethylene, required a high measurement frequency (short relaxation time) due to the rapid mobility of the sidechain, whereas a longer sidechain, such as the vinyl ether sidegroup in Nafion(R), required a much lower frequency (longer relaxation time) to orient the sidechain to a measurable extent. For in-line process studies, a new cylindrical, interdigitated dielectric sensor and associated electronics were developed to make measurements on molten polymers at the exit of an extruder up to 400 degrees C and 3000 lbs/in(2). In-line studies of molten polymers revealed a direct relationship between the dielectric signal and the polymer comonomer composition. The sensor represents a non-invasive, real-time process composition measurement and can be integrated to provide dosed-loop process control.