The sensitivity of an ionization gauge for CH4, C2H4, C2H6, C6H6, and C6H5CH3 has been determined. Results obtained indicate an almost linear increase in sensitivity for these hydrocarbons with the number of electrons per molecule. A reduction in ionization gauge sensitivity was observed after several hours of operation in a hydrocarbon atmosphere. Results obtained here indicate the reduction in sensitivity is due to a high electrical resistance layer formation on the small wire probe ion collector of the Bayard-Alpert ionization gauge. The sensitivity can be restored by electron bombardment of the collector or operating the gauge for several hours in an O2 atmosphere near 10−4 Torr. The high resistance layer buildup can be prevented for many hours by operating the gauge at 250 °C.