Aqueous solutions of alkylamines and 95% ethyl alcohol will react with the acid chloride of polyethylene-co-acrylic acid to form amides and ester, respectively. Chemical and spectroscopic evidence, as well as contact angle measurements, shows that the reactions are due to formation of acid chloride groups below the polymer＇s surface. A film of SOCl2-treated polyethylene-co-acrylic acid retains its reactivity to amines and alcohol even after long soaking in aqueous base. Alkylamines and ethyl alcohol readily penetrate into the polymer film where they react with interior acid chloride groups, while water and charged or large molecules do not. To explain the film＇s properties, a three-layer model of its morphology is proposed. Practical application of the acid chloride film to the tethering of aminoalkyl-5＇-modified oligo-DNA is demonstrated. A site-specific reaction tethers the oligo-DNA to the film exclusively via its aminoalkyl tail. The reaction occurs spontaneously when the acid chloride film is allowed to soak in an aqueous solution of oligo-DNA at pH 11.5. Spectral and contact angle evidence and the reactions of model compounds indicate that the terminal amino group on the modified oligo-DNA reacts with an internal acid chloride group to form an amide bond. A highly active, surface-tethered oligo-DNA results.