The thermomechanical modification of hydrogenated nitrile butadiene rubbers (HNBR) of different molecular parameters was investigated by rheological and light scattering techniques. The influences of acrylonitrile content, degrees of hydrogenation, and Mooney viscosity were examined. A melt blender with Banbury-type mixing blades was used to condition the rubber samples in the temperature range from 190 to 260 degrees C. Light scattering was used to determine the effect of conditioning on MW and hydrodynamic radius of the rubber molecules. Dynamic viscosity (eta') and storage modulus (G') were measured for the as-received and conditioned samples. Experimental results showed that degradation in these rubbers occurred through chain scission and crosslinking. Depending on the molecular parameter, it was found that one of these two mechanisms dominated the degradation process in most brands. Addition of adequate amounts of antioxidants (Irganox 1010 and Irgafos 1.68) was successful only in preventing degradation by crosslinking. Compared to thermal degradation, thermomechanical degradation was found to be much more severe and progressed at much higher rates. Rheology was found to be a very sensitive technique to structural parameters of the polymers and could be used to detect and identify the mechanism of degradation. (c) 2005 Wiley Periodicals, Inc.