Waveguide samples of representative dielectric materials used as the substrates in frequency selective surface (FSS) subreflectors have been measured at -175/spl deg/C with a measurement uncertainty similar to that obtainable at ambient temperature. The samples were measured at X-band frequency using a novel method to overcome the sample test set-up calibration problems at such a low temperature. Full S-parameter amplitude and phase measurements were made and compared to ambient tests performed on the same sample. From the results, the variation in the dielectric constants and the loss tangent of the materials were estimated with the aid of relevant design software. This knowledge has allowed a prediction of the performance characteristics, of an FSS designed and measured at ambient temperature, to be identified at this lower working temperature. Any modifications to the subreflectors electrical parameters will therefore, in theory, be possible so as to fine tune the design for optimum performance at this temperature.