An experimental setup and novel measurement technique are described which allow reliable conductivity measurements to be made at conductivities as low as 10−17Ω−1.cm−1and temperatures up to at least 1300°C. This capability is of particular interest for conductivity measurements on high‐resistivity insulators over a large range of temperatures. This technique has been used to measure the conductivity of single‐crystal alumina from 400° to 1300°C in a 10−7torr (∼1.3 × 10−5Pa) vacuum, equivalent to an oxygen partial pressure of about 10−8torr (∼1.3 × 10−11atm or ∼ 1.3 × 10−6Pa). Surface and gas‐phase conductance are determined as a function of temperature, and the requirements for their minimization are described. A key requirement is a very low voltage between the volume guard and the guarded electrode. The effect of leakage currents due to the sample fixture, electrical feedthroughs, and electronic instrumentation are also evaluated, and proper design features to make the