The effect of trichloroacetic acid-methylene chloride (TCAMC) solvent system on the mechanical properties of poly(ethylene terephthalate) -based flat, high twist, and spun yarns were studied. Pretreatments were carried out in an unrestrained state at different concentrat ions for various times and without employing any heat treatment. The stress-strain behavior of yarns was analyzed. The stress-strain curves showed that the treated yarns exhibit different morphologies. A significant plastic flow region could be detected in treated yarns. The higher interaction between the reagent and the polyester revealed the closeness of their solubility parameter values. Structural modification of yarns due to TCAMC treatment led to an increase in elongation without significant reduction in strength. The reagent caused reduction in initial modulus, yield stress, and an increase in work of rupture. The increase in work of rupture was higher in flat yam but not appreciable in the other two yarns. Statistical analysis of the test data was done.