The translational diffusion coefficient of polystyrene, labeled with azobenzene derivatives and having a molecular weight range 104 ≤ M ≤1.8 × 106, has been studied in matrices of the same polymer in toluene with the technique of forced Rayleigh scattering. The independent variables were the matrix concentration C and molecular weight M of the labeled (tracer) polymer, over a wide range of the matrix molecular weight 104 ≤ Mp ≤ 8.4 × 106. The diffusion coefficient of the tracer polymer Dtrat 20 °C is found to be sensitive to Mp until Mp≥ (3-5)M; when Mpexceeds this limit, Dtrno longer depends on Mpand we call Dtrat such a limit the asymptotic tracer diffusion coefficient Dtr ∞. In the context of the reptation model, the Mpdependence of Dtris interpreted as evidence for a substantial extent of the tube renewal process in semidilute solutions. At the high ends of M (~(1–2) × 106) and C (~ 10–20 wt %), Dtr ∞ ∞ M-3C-3 is a better representation of the data than Dtr ∞ ∞ M-2C-1,75. Further, Dtr ∞is shown to scale with (C/C*) in semidilute solutions for 3.5 × 104 ≤ M ≤ 1.8 × 106.