The conductivity and transport properties of a composite polymer electrolyte were studied by comparing the structure and dynamics as a function of both salt and organic-inorganic composite content. The system consisted of poly(ethylene oxide) (PEO), an organic-inorganic composite (OIC) prepared from aluminum tri-sec-butoxide and [(3-glycidyloxy)propyl]trimethoxysilane (GLYMO) and lithium triflate (LiCF3SO3). The systems with and without salt yielded strikingly different physical properties when the OIC content exceeded 50%. Through analysis of Si-29 NMR spectra, it was found that the lithium ion of LiCF3SO3 (LiTf) promotes the condensation of GLYMO, which peaks near 50% OIC content. Also, short-range structural evidence for PEO-OIC blending at high OIC content was observed in the salt-free system through comparisons of the line shapes of the Al-27 NMR spectra. This blending is absent in the ternary system due to prominent PEO-LiTf interactions, as confirmed by X-ray, DSC, and impedance spectroscopy experiments. Furthermore, the glass transition temperature exhibits a linear increase as a function of OIC content, whereas the conductivity over this range first shows a sharp increase followed by a mild decrease. The dielectric constant also was found to vary nonlinearly with OIC content, indicating that ionic screening is modulated by OIC. Because in this system the conductivity and the glass transition temperature do not show a significant correlation, although structurally it is clear that PEO and salt are intimately mixed, a model was developed for the transport that focuses principally on the density of mobile lithium ions. The model predicts relatively constant ion mobilities and diffusion constants but a strongly varying mobile ion number density as a function of OIC content, which then explains the dependence of conductivity on OIC content in this electrolyte.