In this paper, two frequency reconfigurable antennas using the ON/OFF states of a microelectromechanical system (MEMS), are presented. A multifrequency planar inverted-F antenna (PIFA) is designed with a L-shaped open slot upon its main plate. As a proof-of-principle, an ON state RF MEMS switch, modeled by a piece of copper (1x1 mm/sup 2/), is inserted into the slot to achieve frequency switching (while OFF state is modeled without the piece of copper). According to the switch position alone the slot, the GSM operating frequency is not detuned while the other standards become reconfigurable. To validate our concept, a thermal MEMS switch, designed and developed at CEA-LETI is used. The MEMS switch is presented and its scattering parameters are measured and compared with simulated results from an equivalent RLC circuit. The active device is then inserted in the slot to valid the simulated frequency behaviour of the PIFA. Based on the same concept, a planar antenna is designed. It consists in a printed PIFA on a high-resistivity silicon substrate (10 k/spl Omega/.cm). The antenna resonates in the 2.4 GHz WLAN band but the insertion of two small gaps in its main aim which model the OFF state of a MEMS switch allows the antenna to work in the 5.150-5.875 GHz HIPERLAN 2 frequency bands.