Two cold cathode discharge gauges have been calibrated from 10−4 to 10−12 Torr for argon and hydrogen and to 10−10 Torr for helium, nitrogen, and carbon monoxide. With minor exceptions the sensitivity decreases with pressure; for example, the sensitivity for argon for one gauge was ∼3.5 A/Torr at 10−4 Torr, ∼1.5 A/Torr at 10−10 Torr, and ∼0.1 A/Torr at 10−12 Torr. The calibration may be represented by a series of linear segments which probably correspond to stable discharge modes in the gauge. In addition, overlap of stable modes occurs and hence the accuracy of pressure measurement is difficult to predict even with a detailed calibration of a specific gauge.