A technique to distinguish between materials that can and cannot shield microwaves in liquid nitrogen was developed. The samples are placed in a modifiedX‐band waveguide, which operates by the same ‘‘insertion loss’’ principle as a coaxial device used to measure shielding effectiveness from 300 to 1000 MHz. Verification of the waveguide fixture was accomplished by measuring the shielding effectiveness of a good conductor, coppper; a nonconductor, Teflon; and two intermediate conductors, steel and a carbon‐filled elastomer. After verification, the waveguide fixture was used at liquid nitrogen temperatures to compare the shielding effectiveness of YBa2Cu3O7−x, a high temperature superconductor, with copper, a known shielding material.