The main feature of amorphous materials is the presence of excess vibrational modes at low energies, giving rise to the so-called “boson peak” in neutron and optical spectroscopies. These same modes manifest themselves as two-level systems (TLSs) causing noise and decoherence in qubits and other sensitive devices. Here, we present an experiment that uses the spin relaxation of dangling bonds at the Si/(amorphous) SiO 2 interface as a probe of TLSs. We introduce a model that is able to explain the observed nonexponential electron spin inversion recovery and provides a measure of the degree of spatial localization and concentration of the TLSs close to the interface, their maximum energy, and its temperature dependence.