The accurate and precise measurement of process stream temperatures during injection molding can be difficult, since the cyclic operation results in spatial and temporal variations of the stream temperature. This paper examines the application of a spectral infrared (IR) pyrometer to monitor the cooling of a polymer melt within the mold cavity during a typical injection molding cycle. An outline for interpreting the radiation signal collected with the IR pyrometer is presented. The discussion includes theoretical aspects as well as experimental results. The theoretical approach accounts for the polymer transparency (attenuation behavior) at the spectral wavelength of the pyrometer and also for the temperature gradient within the polymer, thereby establishing the concept of a critical depth for a given pyrometer/polymer combination. The final analysis reveals good agreement between the predicted and measured results for the transient cooling conditions of the polymer within the mold cavity. Depending on the degrees of polymer transparency used in the theoretical prediction, the deviation between the measured and predicted transient bulk temperatures after mold filling (during the mold cooling stage) varies from +/-2 degrees K to +/-9 degrees K.