A theoretical basis for the prediction of the properties of mixtures of virgin and reprocessed polymers is presented. The concept is developed using equations that describe the loss of polymer properties as a function of processing cycles. These equations correspond to properties that are important in characterization and quality control, such as melt flow index and tensile and impact strengths. Experimental data were determined and used to test the models derived. For glass-fiber-reinforced thermoplastics, the data on the degradation of fiber lengths and mechanical properties can help to understand the effect of reprocessing on the fiber-matrix interaction.