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Porous PTFE films were corona charged with high voltages at room and elevated temperatures and their direct and inverse piezoelectric coefficients were determined. In both cases, piezoelectric d/sub 33/ coefficients of up to 600 pC/N were found. These values are more than one order of magnitude higher than those of conventional piezoelectric polymers such as poly(vinylidene fluoride). They are comparable to the coefficients of inorganic piezoelectrics and of the recently developed porous polypropylene electrets, but have the added advantage of a much better thermal stability.