An oligodeoxynucleotide (ODN) conjugated to poly(ethylene glycol) (PEG) through a pH-responsive ester linkage (PEG-ODN conjugate) was successfully synthesized by the Michael reaction of 3'-thiol-modified ODN with a heterobifunctional PEG bearing an acetal group at the alpha-end and an acrylate group at the omega-end (acetal-PEG-acrylate), aimed at the development of a novel ODN delivery system. The prepared PEG-ODN conjugate and linear-poy(ethyleneimine) (L-PEI) spontaneously associated to form a polyion complex (PIC) micelle whose diameter and polydispersity index micro(2)/Gamma(2)) were 102.5 nm and 0.096 as determined by DLS measurements, respectively. Both the PEG-ODN conjugate and PIC micelle showed cleavage of the ester linkage at the endosomal pH (=5.5), suggesting that the PIC micelle is anticipated to release the ODN in the intracellular compartment. Furthermore, the PEG-ODN conjugate in the PIC micelle was stable against deoxyribonuclase (DNase I) digestion and has no interaction with the serum component because of the steric stabilization of the highly dense PEG corona surrounding the PIC core. These characteristics of the PIC micelles entrapping the PEG-ODN conjugate are promising for their utility as a novel ODN delivery system.