A phase switch based on switching between two twoport passive networks with a phase difference of 180º is reported. Two RF MEMS single-pole-double-throw structures are used to select the phase path. The fabrication is made on both highresistivity silicon and quartz substrates using the eight-mask FBK-irst surface micromachining process. The measured phase shift is 180º±5º in the frequency range 14−20 GHz on quartz wafers and in the range 8−20 Ghz on silicon wafers.