Phosphoric acid doped cellulose acetate propionate (CAP) consisting of poly(ethylene glycol) (PEG) as plasticizer was investigated. Ionic conductivities and dielectric studies were carried at different temperature with varying concentration of H3PO4 using AC impedance method. The highest conductivity was 8.1 x 10(-4) S cm(-1) at 343 K and a long tail was featured in dielectric studies indicating good capacitance nature of the electrolyte. Interactions between added constituents were observed in FTIR and differential scanning calorimetry studies. Thin and compact fabricated supercapacitor demonstrated specific capacitance of 64 F g(-1) using cyclic voltammetry. Furthermore, the supercapacitor properties like AC impedance and charge-discharge were studied. Stability was up to 96% at 1000th cycle. (C) 2015 Society of Plastics Engineers