A fast diffusing As species in ion-implanted SiO2 in N2 annealing ambient has been observed for the first time. This As diffuses 20X faster at 1100°C than the slow As found in oxidizing ambients. The activation energy of diffusion of this fast As is 0.71 eV, in sharp contrast to 4.3 and 4.0 eV for the slow As in O2 and O2 /H2O, respectively. The fast species is believed to be As occupying either the O-site or interstitial site in the SiO2 network. By assuming a fast and a slow species coupled together by transitions, we were able to obtain reasonable fits to the SINS diffusion profiles.