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In order to provide a rational basis upon which future developments in nonaqueous finishing and other processing of textile materials may be based, a basic study is being undertaken of the interactions between textile fibers and a wide spectrum of organic solvents. In this paper are reported the effects on mechanical properties of a polyester yam of 26 organic solvents differing widely in chemical and physical characteristics. The yams were conditioned in each solvent for 16 hr at 21°C and their load-extension behavior determined in the solvent. While many solvents were found to have only minor effects on the mechanical properties, several solvents caused major changes in the shape of the load- extension curve of the polyester yam. The active solvents caused a decrease in the initial modulus, a decrease in the yield stress, and an increase in extensibility. In several cases a significant plastic flow region could be noted. Dioxane, acetone, trichloroethylene, tetrachloroethane, methylene chloride, nitrobenzene, nitromethane, acetonitrile, and di methylformamide (DMF) were among the most active solvents. These solvents also caused a major irreversible longi tudinal shrinkage of the yarn, and the mechanical properties of the yarn were only partially recovered after removal of the solvent and reconditioning to a standard condition.The interactions between the solvents and the polyester yarn, as estimated by the decrease in the yarn elastic modulus, were correlated with the solubility parameter δ of the solvents. It was observed that maximum interactions take place with solvents whose solubility parameter approaches that of polyethylene terephthalate δ=10.7. There is some indica tion that these strongly interacting solvents fall into two groups: one with δ values ranging from 9 to 10, the other with δ values ranging from 11.5 to 13. It is suggested that these ranges may reflect specific solvent interactions with the aro matic and aliphatic segments of the polyester molecules. [ABSTRACT FROM PUBLISHER]