The dyeing behavior of poly(ethylene terephthalate) based Aat, high twist, and spun yarns pretreated with trichloroacetic acid-methylene chloride (TCAMC) reagent was studied. Disperse dyes having two different energy levels were employed for the dyeing work. The effect of time, temperature, and dye diffusion transition temperature (T-D) on dye uptake was analyzed. A considerable increase in equilibrium dye uptake and decrease in T-D Of all the treated yarns were observed. The variations in dye diffusion behavior of higher and lower molecular weight dyes and the difficulties encountered in calculating the diffusion coefficient of the dyes are discussed. The structural and morphological changes effected by the pretreatment were also investigated using XRD and SEM, respectively. The increase in lateral order of the treated yarns was noted. The possible reason for an unusual relationship between the increase in lateral order and increase in dye uptake was explained. The cross-sectional shape and swelling and the smoothening out of the fiber surface were evidenced by SEM.