The excellent frequency stability and cryogenic environment of a superconducting resonant cavity provides a sensitive method for observing trap‐filling in CdS and similar materials. When used with thermally stimulated conductivity and dc photoconductivity, it is possible to solve for trap energy, capture cross section, density of trap states, and free‐carrier lifetime. The technique is that used by Arndt, Hartwig, and Stone to observe optically induced changes in the complex dielectric constant by inertia forces on free carriers in Si and other indirect‐gap semiconductors. Using TSC, pure CdS crystals showed very weak trapping effects and CdS: Al displayed electron traps at 0.52, 0.35, and a group at 0.265, 0.20, and 0.15 eV. A quenching, or hole trapping, level was seen at 1.6 eV below the conduction band. Deep electron trap densities were about 1015cm−3and the shallow set was about 1017cm−3each. Hole trap density was slightly in excess of 1018cm−3. In CdS: Al, the photodielectric frequency shift of an 840 MHz cavity was proportional to the integral of photon flux, indicating the effect was caused by polarization of electrons in traps. Photodielectric data yielded an effective trap energy for the three shallow levels at 0.176 eV and a total density of 3×1017cm−3. Density of trapped electrons is calculated from frequency change. Capture cross section for the shallow levels was about 10−14cm2. Additional data from dc photoconductivity provide free‐carrier lifetime and location of the electron Fermi level as a function of filled trap density. CdS:Al displays a ``tap'' effect which can erase the accumulated frequency change without warming.