Measurements of the translational diffusion coefficients of poly(propylene oxide) molecules in the melt as a function of molecular weight just above the critical molecular weight for entanglement indicate that constraint release is a significant mechanism for diffusion. Diffusion coefficients of labeled polymer chains in molten poly(propylene oxide) have been measured by fluorescence redistribution after pattern photobleaching both below and above the critical molecular weight for entanglement. Labeled poly(propylene oxide) was synthesized with exactly one fluorophore per molecule. The diffusion coefficients of labeled chains were found to depend upon the length of solvent chains for solvent molecular weights up to four times the critical molecular weight for entanglement. © 1985, American Chemical Society. All rights reserved.