In recent years, experimental studies [1-3] have suggested that Si interstitials may play a role in facilitating As transient enhanced diffusion during pn junction formation in silicon. These studies contradict conventional models that assume vacancy-mediated As diffusion. Using density functional theory calculations within the generalized gradient approximation, we have examined the structure, stability, and diffusion of the neutral As-Sii pair. We find the lowest energy structure is comprised of an As and Sii atom pair that is aligned in the  direction while sharing a lattice site. We have calculated the binding energy as well as diffusion pathways and barriers for the neutral As-Sii pair. Our results suggest that the neutral As-Sii pair has a binding energy relative to neutral Sii and neutral As of 0.63 eV. We also find an overall diffusion activation energy of 3.3 eV, which is similar to experimental observations for As diffusion and preVious calculations for As-vacancy complex diffusion. These results clearly support that interstitials can contribute significantly to As transient enhanced diffusion, especially in regions where interstitials exist in excess. In addition, interstitial-mediated arsenic diffusion suggests that interstitials may also play a role in arsenic agglomeration.