I will show how spectral features in a stochastic gravitational-wave background can be used to probe the properties of long-lived beyond-the-Standard-Model particles. Such particles can induce a temporary period of early matter domination, leaving characteristic frequency scales in a primordial gravitational-wave spectrum that encode the onset and end of this epoch. For freely propagating backgrounds, these features determine the particle decay rate and abundance-weighted mass independently of the microphysical origin of the gravitational-wave source. For actively sourced backgrounds, the end of early matter domination still provides a source-independent probe of the decay rate, while the second feature becomes source dependent. Once a particle model and production mechanism are specified, these observables can be translated into particle masses and microscopic couplings. I will illustrate this framework with examples including right-handed neutrinos and gravitinos.