Poly(4,4’-cyclopentylidene diphenylene toluene-2,4-disulfonate) (PSBPT) has been synthesized by conventional interfacial polycondensation of 1,1’-bis(4-hydroxyphenyl) cyclopentane (0.005 mel) and toluene-2,4-disulfonyl chloride (0.005 mol) using water (50 ml)-chloroform (12.5 ml) as interphase, alkali (0.015 mol) as acid acceptor and cetyltrimethyl ammonium bromide (0.125 g) as emulsifier. The structure of PSBPT has been supported by infra-red and nuclear magnetic resonance spectral data. A 40 mu m thick film has 200.1kg cm(-2) tensile strength and 0.6% elongation at break. A 0.19 mm thick film has static hardness 14.5-16.5kg mm(-2) at different loads (15-60g). The density (1.353 +/- 0.0009g cm(-3)) of PSBPT has been determined by the floatation method and compared with calculated values (1.3973g cm(-3)). Thermogravimetric (t.g.a.) and differential thermal (d.t.a.) measurements have been made at four different heating rates and 10 degrees C min(-1), respectively in N-2 atmosphere. D.t.a. showed the glass transition temperature T-g at about 134 degrees C and two exotherms at 337 degrees C and 362 degrees C which might be due to decomposition. PSBPT is stable up to about 355 degrees C in N-2 atmosphere and involves two-step degradation. The average energy of activation, pyrolysis order and frequency factor for step-I are 52.6 kcal mol(-1), 2.5 and 9.22 x 10(13) min(-1), respectively. PSBPT has excellent hydrolytic stability towards acids and alkalis.