An experimental study of the rheological properties of talc, calcite, and mixed particle compounds of polystyrene (PS) compounds has been carried out on a range of instruments at 200 degrees C. We investigated the shear viscosity over a very wide range of shear rates in capillary, cone-plate, and sandwich rheometers. Shear stresses below which there is no flow, i.e. yield stresses, were found. A uniaxial elongational rheometer based on floating samples on a silicone oil bath was used to study the uniaxial extensional flow characteristics. The compounds were found to absorb silicone oil. It was, however, possible to estimate yield values of highly filled compounds in uniaxial extension by investigating the stretching of vertical filaments in a chamber filled with nitrogen.