Hydrogenated amorphous dielectric thin films are critical materials in a wide array of technologies. In this work, we present a thorough investigation of the thermal conductivity of hydrogenated amorphous silicon nitride (a−SiNx:H), a ubiquitously used material in which the stoichiometry plays a direct role in its functionality and application. In particular, through chemical, vibrational, and structural analysis in tandem with thermal conductivity measurements on chemically variant silicon nitride films, we show that hydrogen incorporation into silicon nitride disrupts the bonding among silicon and nitrogen atoms, and directly impacts the thermal conductivity, leading to as much as a factor of 2.5 variation in heat transfer. This variability, driven by the change in hydrogen content, is fundamentally related to the changes in the average atomic distances, as we experimentally measure with selected-area electron diffraction and computationally show with molecular dynamics simulations. This, combined with our evidence of chemical and spatial fluctuations on the order of average atomic pair distances, leads us to conclude that the vibrational heat transport in a−SiNx:H is primarily dominated by diffusonlike modes. The results presented in this work combined with our extensive review of prior reports on the thermal conductivity of a−SiNx:H films resolves discrepancies in decades of prior literature and facilitates a more universal understanding of the vibrational heat transport processes in hydrogenated amorphous silicon nitride.Read less <