Observations of the influence of the electron emission current, the electron collector potential, gas composition and wall cleanliness upon the bistable operation of the Bayard-Alpert gauge are reported. This bistable behaviour is observed as a duality of manometric sensitivities and ion pumping speeds and the magnitude and variation of these with electron collector potential was measured in both modes of operation for several gases. It is concluded that it is advisable to operate gauges at electron collector potentials less than 250 V, to avoid instability and maintain optimum sensitivity.