The trigger discharge gauge has been compared with a thoroughly processed Bayard-Alpert ionization gauge capable of measuring pressure to2×10−11Torr. Differences no greater than a factor of two were observed between the two gauges for pressures between1×10−11and10−4Torr. By subtracting the x-ray limit of2×10−11Torr, the ionization gauge readings were corrected and indicated fair agreement with the trigger discharge gauge down to1×10−11Torr. Comparison of these gauges with a partial pressure analyzer indicates the ionization gauge output decreased linearly down to10−10Torr and saturated at2.6×10−11Torr while the trigger discharge gauge current varied asP1.2in the range between10−7Torr to5×10−12Torr—the lowest indicated pressure achieved while operating the partial pressure analyzer. It is concluded that slow and continued ion desorption and outgassing of the ionization gauge is responsible for differences of more than a factor of two in trigger gauge and ionization gauge readings.