A new technique using a multi-port annular slot antenna for the measurement of complex permittivity is developed. The benefit of this now technique is that a two port network can be used on a planar device. The forward case of measuring S/sub 21/ is combined with a simple but effective calibration technique to compensate for the feed network. The validity of the method is confirmed by using methanol and ethanol, which are commonly used reference materials, for comparison between expected and measured results.