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The ac and dc insulation resistance of two open-circuited, commercial, standard platinum resistance thermometers is investigated over the range 0-750 °C and 2-4000 Hz, to determine the magnitude of temperature errors caused by the insulation shunt across the platinum sensor. It is found that ac insulation leakage is always greater than dc leakage; that ac leakage increases as the test frequency increases; and that both ac and dc leakage usually increase rapidly as the temperature increases above 500 °C. For example, the temperature errors at 630 °C caused by insulation leakage when a thermometer is measured with dc, 100 Hz and 1000 Hz excitation can reach about 1, 2 and 8 m °C respectively. With a metal-sheathed thermometer, it is confirmed experimentally that the more readily measured insulation resistance between the sheath and the leads gives a useful approximation to the true shunt across the sensor at high temperatures. Similarly, it is shown for a quartz-sheathed thermometer that the insulation resistance between the leads and a metal mesh placed over the sheath will also approximate the true shunt at high temperatures. Other properties investigated include: dc leakage as a function of the thermometer furnace immersion; ac leakage in a thermometer containing moist air; ac leakage in the external thermometer leads; and the ac capacitance of the thermometer and leads.