The ionic states of the single-ion conductor, poly(oligo-oxyethylene methacrylate-co-alkali metal acrylamidocaproate) ((CH2CCH3COO(CH2CH2O)(9)H)(x)-(CH2CHCONH(CH2)(5)COO-M+)y), M+ = Li+, Na+, K+, were studied. The dependencies of glass-transition temperature and ionic conductivity on cations were also investigated. The existence of the ion aggregates, ion pairs and free ions in the single-ion conductors was confirmed by FTIR spectra. The peaks from the ion aggregates were more clearly found for the smaller cation, which is due to the stronger ion-ion interaction between the ion pairs for the smaller cation. The glass-transition temperatures increased with increasing the size of the cation, which indicates that the mobility of chain is mainly restricted not by the ion-ion interaction between the ion pairs, but by the ion-dipole interaction between the free ion and the PEO. It is expected that only the nearby ion pairs are involved in the formation of the ion aggregates without severe stretching of the main chain.