A low‐temperature material‐properties analyzer, using a superconducting microwave resonant cavity, is discussed. Placing semiconductor or dielectric material samples in the cavity perturbs the resonant frequency, absorbed microwave power, and cavityQ. Additional perturbations occur when the sample complex dielectric constant is altered by a thermal, nuclear radiation, or optical stimulus. In samples such as Si, GaAs, CdS, and CdTe, these perturbations have been used to determine such material properties as relaxation time, lifetime, Fermi level, trap ionization energy, trap density, capture cross section, free‐carrier density, and trap population. A contactless experimental technique similar to the thermally stimulated conductivity experiment is proposed. The contactless ac measurement system is shown to be sensitive, accurate, useful with randomly shaped or powdered samples, and applicable to many types of insulators and semiconductors.