Nanocomposite solid polymer electrolytes (SPEs) have been prepared from polyethylene oxide (PEO), organically modified nanoclay (MNclay), and tetraethylammonium tetrafluoroborate (TEABF(4)) salt. The concentration of the salt has been varied in the respective SPE, wherein PEO/MNclay ratio was kept constant. It has been proposed that three types of complex formation could be operative in the SPEs due to the interactions among PEO, MNclay, and the salt. The complex formation mechanism has been postulated on the basis of X-ray diffraction (XRD) analysis, transmission electron microscopic (TEM) observation, differential scanning calorimetric (DSC) analysis, and polarized optical microscopic (POM) observation. Complex 1' and complex 3' formation could be involved in the crystalline phase as indicated by DSC and XRD analyses, whereas complex 2' formation might be restricted in the amorphous phase as suggested by TEM observation. The ionic conductivity of the SPEs has been correlated with the results obtained from XRD, DSC, and POM analyses. The formation of complex 1 and complex 2 could be responsible for the increase in the ionic conductivity, whereas complex 3 formation might decrease the ionic conductivity. An activated carbon-based supercapacitor has been fabricated using SPEs and characterized by cyclic voltammetry, galvanostatic charge-discharge' behavior, and impedance spectroscopic analysis. POLYM. ENG. SCI., 55:1536-1545, 2015. (c) 2015 Society of Plastics Engineers