This thesis discusses RF dielectric spectroscopy of fluids using microstrip line and coplanar waveguide as well as the use of RF fluids in passive RF circuits. The use of liquids as a tuning element and as a coolant is not very well explored. In this work different liquids are analyzed for its dielectric properties and its tuning capability. Several passive tuning circuits have been designed and developed, for example a tunable bandstop filter that uses liquids to shift the λ/4 resonance modes of the stub. In addition a patch antenna with the tunable liquid stub that uses the open end effect of the stub as a switch is fabricated and studied.