Poly(N-methylglycine) (NMG) and poly(N-ethylglycine) (NEG(n)) obtained by polymerization reactions initiated by benzylamine have no carboxy termini, such as those in normal but have only amino termini, which exist primarily as cations in aqueous media at a pH value of ca. 9.5, observed in aqueous solutions without any buffer reagents. Therefore, polypeptoids, such as NMG(n) and NEG(n), possessing a degree of polymerization (DP) higher than a certain value behave as cationic monopolar polymeric chain molecules in aqueous solution. It has not been clarified so far whether such a monopolar chain molecule exhibits dielectric relaxation (DR) behavior resulting from its molecular motions in aqueous media as dipolar chain molecules. DR measurements revealed that NMG(19) and NEG(17), possessing DPs of 19 and 17, respectively, dissolved in pure water dearly demonstrated pronounced DR behavior caused by fluctuating molecular motions of cationic termini at relaxation times of ca. 4 and 9 ns at 10 degrees C (283 K). The hydration numbers of NMG(19) and NEG(17) per monomeric residue (n(m)) in aqueous solution were also evaluated via DR data as functions of temperature, and the nm value of ca. 4.5 at 10 degrees C showed a remarkable reduction to ca. 2.0 around 40 degrees C (313 K) and 30 degrees C (303 K), depending on differences in the substituted group: methyl and ethyl groups. This temperature-dependent hydration/dehydration behavior found in NMG(19) and NEG(17) slightly influenced the sizes and molecular dynamics of the monopolar chain molecules in aqueous solution.