Infrared (IR) thermosensors that measure the temperature of polymeric melts in processing equipment by guiding the thermal radiation along optical fibers to the optovoltaic transducer are on the market. The fundamental concepts of thermal radiation must be viewed in a volumetric sense. In sequence, a model is presented to estimate the response of such IR thermosensors. It is dictated by the volumetric absorptivity of the polymer and the transmittance of the optical fiber. For typical commercial IR thermosensors, the entire wavelength range between 1.5 and 2.2 mu m is relevant. Penetration depths can vary from magnitudes of mm to dm, since the IR probe is sensitive to the maximum temperatures in its acceptance volume. These findings were put to the test in two different experimental settings. In the first, the calculations overestimated the comparable experimental values by 10 to 20 degrees C, but were reasonable given the relatively simple experimental apparatus. In the second setup, the maximum temperature of melt coming off the screws, measured by penetrating a small thermocouple into the stream, matched the IR temperature sensor’s readings. The measurement of temperature excursions in different mixing geometries is demonstrated. No kneading section with forward staggered paddles gave significant increases in temperature. In contrast, maximum temperature excursions could be measured when reverse elements were in place.