Polycrystalline silicon diffusion sources have been analyzed using secondary ion mass spectrometry. Polysilicon films were grown by standard low‐pressure chemical vapor deposition and doped with As, P, or B by ion implantation. Although dopant segregation at the poly‐Si/single‐crystal‐silicon interface occurred and has been analyzed quantitatively, no diffusion barrier has been observed at this interface. Diffusion profiles in the single‐crystal substrate have been measured for diffusion temperatures between 800 and 1000 °C. It is shown by comparison to numerical simulations that these profiles obey standard diffusion theory. Proper boundary conditions to be used for such calculations are deduced for the different dopant species. Furthermore, the saturation concentration of boron in silicon over the temperature range given above has been reliably determined.