The decay of radioactive isotopes embedded into detectors can be characterized with high accuracy if all decay products are captured inside the detector and each decay is measured with ~100% efficiency. This approach is especially promising when a cryogenic detector with high energy resolution is used that can separate signals from different isotopes or different decay channels. We are developing cryogenic detectors with operating temperatures of ~0.1 K for high-resolution measurements of nuclear decays. This talk will discuss accurate decay measurements of Be-7implanted into superconducting tunnel junction (STJ) detectors in the context of a search for hypothetical sterile neutrino particles. We will also discuss work on accurate isotope ratio measurements of small actinide particles that are embedded into magnetic microcalorimeter (MMC) radiation detectors with an energy resolution <5 keV at several MeV. In both cases, the experimental accuracy depends on the counting statistics of the measurement and on precise knowledge of the detector response function.