We have found that encapsulated thin Si films, adjacent to SiO2, are dissolved at elevated temperatures by a reaction: Si+SiO2→2SiO, with SiO diffusing through the silica network. The presence of SiO in SiO2 in a concentration ≤3 at. % has a profound effect on thermomigration of dopant impurities in the oxide. We propose a model for transport of As‐, Sb‐, P‐, and Ge‐rich precipitates, 50–1000 Å in diameter, based on a reaction between SiO2 and the precipitates that is mediated by a catalytic influence of SiO molecules. Only in the presence of SiO are the bonds in the SiO2 network broken ahead of the drifting clusters and regrown behind them. The model predicts that the migration of precipitates is controlled by diffusivity of SiO in silica, consistent with the narrow range of drift velocities for clusters of different chemical compositions. The data also suggest that SiO2 dissociation at the Si/SiO2 interface is diffusion limited, which explains why decomposition of thin SiO2 in vacuum is spatially inhomogeneous, while thick films used in our experiments react uniformly.