Near-infrared evanescent-wave cavity ring-down spectroscopy (CRDS) has been applied to study the defect evolution in an amorphous silicon (a-Si:H) thin film subjected to a directed beam of atomic H with a flux of (0.4–2) × 1014 cm−2 s−1. To this end, a 42 ± 2 nm a-Si:H film was grown on the total internal reflection surface of a folded miniature optical resonator by hot-wire chemical vapor deposition. A fully reversible defect creation process is observed, with a nonlinear dependence on H flux, with a time resolution of 33 ms and a relative sensitivity of 10−7. Using polarizing optics, the CRDS signal was split into s- and p-polarized components, which, combined with E-field calculations, provides depth sensitivity. Extensive kinetic modeling of the observed process is used to determine rate constants for the hydrogen–material interactions and defect formation in a-Si:H, as well as revealing a high diffusion coefficient for atomic H on the order of 10−11 cm2 s−1. A novel reaction pathway is proposed, whereby H inserted into weak Si–Si bonds recombines with mobile H, resulting in a limited penetration depth for atomic H from the gas-phase on the order of 10–15 nm.