Diffusion of boron from ion implanted polycrystalline silicon source through 12.5 nm oxides was measured as a function of annealing ambience and doping concentrations of phosphorus or arsenic in the polycrystalline silicon. For comparison, boron ion implanted into 500 nm oxides was also investigated. In both cases, annealing in forming gas (90% N2, 10% H2) increases the diffusion rate of boron over that in nitrogen and the rate increase is the result of a large increase in the pre-exponential term. Diffusion of ion implanted boron in oxides is much slower than that from an ion implanted polycrystalline silicon source. The lowering of the rate is the result of the five orders of magnitude decrease in the pre-exponential term, even though concurrently there is also a decrease in the activation energy. Co-doping of polycrystalline silicon with either phosphorus or arsenic lowers the pre-exponential term by approximately 10%. Therefore, we conclude that boron diffusion through oxides is entropy dominated.