Electron multipliers are capable of emitting significant volumes of various gases while they are in operation at low pressures. Errors in ultrahigh-vacuum measurements may result. Experiments are described which show that, in a typical case, most of the evolved gas comes from the dynode and collector surfaces as a result of electron bombardment. Additional gas evolution may occur as a result of dynode resistor heating, if internal resistors are used. It appears that gas may also be released from some kinds of insulators when voltage is applied. Methods of minimizing these effects are discussed.