For the first time it is shown that the lifetime of capacitive RF MEMS switches depends on the ambient gas pressure. A change of the pressure causes a change of the electric strength of the gas and as a result electric discharging during the operation can occur. This indicates that insulator charging, the main failure mode in these switches, probably not only occurs upon contact between the top electrode and the insulator, but also without contact, due to electron emission or electrode-gap breakdown.