The effect of pH, temperature, dye concentration, and additives on the absorption of the dyes FD & C (Food, Drugs and Cosmetics) Blue 1 and Blue 2 upon poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) subjected to different draw ratios (and, thus, resulting in different percentage crystallinities determined with DSC and wide-angle X-ray diffraction patterns) was investigated and a correlation between these parameters and the dye uptake was suggested. The absorption kinetics of the dyes on the polyesters were studied and a mechanism based upon the development of hydrogen bonds between the dyes and the end groups (-COOH, - OH) of the polyesters and the swelling of the network was proposed. Finally, an indirect measurement of the T(g) values of the polyesters was suggested based upon the different dyeing diffusion rates.