The diffusion of boron and phosphorus in heavily doped layers in silicon (extrinsic at process temperatures) has been studied to obtain information on silicon interstitial concentrations in heavily doped layers. Diffusion of both dopants in n- and p-type isoconcentration structures has been measured at a number of temperatures and for two surface conditions : a surface passivated with low pressure chemical vapor deposited oxide/nitride, and for a growing thermal oxide. The amount of oxidation-enhanced diffusion (OED) has been measured for both dopants and was found to be smaller than for the same anneals under lightly doped conditions (intrinsic at process temperatures). Phosphorus data has been analyzed, and effective silicon interstitial energy levels-useful in modeling the diffusion of heavily doped layers-have been extracted. Phosphorus diffusion has also been examined in isoconcentration structures with and without a lightly doped epitaxial layer at the surface, to study the effect of doping at the silicon-silicon dioxide interface on diffusivity enhancements. Here it was found that samples with the expitaxial layer had larger enhancements compared to samples that did not. This is explained as the result of charged interstitial drift in the electric field set up by the ionized dopant atoms, and the resulting increase in the flux of interstitials away from the oxidizing interface.