Proton-conducting polymers were prepared by sulphonation of polyindene (PInd) with chlorosulphonic acid with various degrees of sulphonation (DSs) and characterised by C-13 and H-1 NMR, FTIR spectroscopy, TGA, DSC, and IES. The NMR spectra of the sulphonated polyindene (SPInd) showed only the mono-ring-substituted product at the meta-position. The SPInds are hydrophilic, and their water uptake increased with increasing DS. The SPInds exhibited 10 to 17% bound water, and samples with DSs of 21 and 96% exhibited conductivities of 10(-4) and 10(-2) S.cm(-1) at room temperature, respectively. (C) 2014 Elsevier B.V. All rights reserved.