A molecular-level understanding of ion and polymer dynamics in imidazoliumpoly(ethylene glycol) (PEG) copolyester ionomers having different counteranions [PF6- vs (CF3SO2)(2)N-], alkylene spacer lengths [(CH2)(6) vs (CH2)(11)], and PEG segment lengths [(CH2CH2O)(22) vs (CH2CH2O)(44)] is investigated for the development of fast single-ion conducting materials, using dielectric relaxation spectroscopy and X-ray scattering. The variations of the counteranion size and PEG segment length lead to substantial changes in simultaneously conducting counteranion content (p), their mobility (mu), ion (alpha(2)), and segmental (alpha) relaxations, with consequences for ion transport. However, there is no significant influence of the alkylene spacer length on the counteranion conductivity (sigma(DC)). Imidazolium-PEG copolyester ionomers with larger (CF3SO2)(2)N- (= Tf2N) counterions show higher p with lower activation energy and higher mu with lower Vogel temperature as well as fast alpha(2) and alpha motions with lower glass transition temperature, resulting in higher sigma(DC). For the PEG segment length, the longer PEG segment leads to a copolyester crystallization, reflected by the observation of distinct X-ray diffraction peaks and a Maxwell-Wagner-Sillars interfacial polarization, bringing out an abrupt decrease in sigma(DC) at the phase transition. On the other hand, imidazolium-PEG copolyester with the shorter PEG segment has higher conducting counteranion fraction (p/p(0)), mu, and static dielectric constant (epsilon(s)) compared to the similar polyester analogue with no PEG segment. Incorporation of the PEG segments with an optimized length into the main chain polymer plays an important role in directly boosting counteranion conductivity of the single-ion conducting copolyester ionomers.