Researchers characterized the dye diffusion properties of PET, focusing on the influence of molecular orientation in the amorphous region of the fibers. Tests employed a recently developed technique in which the natural fluorescence of the PET molecular chain was used as a probe to determine orientation in noncrystalline areas. The fluorescing unit was a randomly occurring sandwich dimer formed from overlapping terephthalic molecules in adjacent polymer chains. Fiber specimens were dyed in an aqueous dyebath at 90 degrees centigrade with the azo dye C. I. Disperse Orange 3. Acetate buffers maintained the dyebath at pH 5.0. Lambert-Beer relationships for methylene chloride solutions of the recrystallized dye were found to be linear and were therefore used as calibration curves for dye uptake. Results showed that the dye diffusion coefficient decreased rapidly with increases in drawing speed over the range from 1,500 to 3,500 meters per minute, and then leveled off before increasing slightly over the range from 6,000 to 7,000 meters per minute. Changes in molecular mobiltiy and amorphous orientation due to drawing speed correlated with changes in dye diffusion. 6 refs.