The sulphonation of commercially available PEEK in powder form (Gatone(TM), Gharda Chemicals Limited, India) was carried out using conc. H2SO4 under different reaction conditions. The duration of reaction was varied from 3-5 h, polymer concentration 4-10% (w/v) and temperature 35-50degreesC. Structural characterisation of sulphonated polymers was done by elemental analysis, FT-IR and H-1-NMR spectroscopy. The degree of sulphonation as calculated from H-1-NMR and elemental analysis (S-content) was found to be in the range of similar to50-80%. Multistep mass loss was observed in thermogravimetric traces (recorded in N-2 atmosphere). The first step (50-225 +/- 25degreesC) was due to loss of moisture (1-10%) and second step (250-425 +/-25degreesC) has been attributed to volatilization of SO3 from the sulphonic group. The backbone degradation takes place above 450degreesC. The mechanical properties and proton conductivities of various sulphonated samples was also evaluated. (C) 2005 Springer Science + Business Media, Inc.