Arsenic implanted into thermally grown SiO2 was diffused in ambients of O2 and N2, and arsenic distributions in the SiO2 were analyzed with secondary ion mass spectrometry (SIMS). Arsenic was implanted at energies between 40 and 70 keV with a dose of 1×1015 cm-2 and diffused at temperatures between 1000 and 1100 °C. The profiles revealed retarded diffusion in the high‐concentration region (≫1×1019 cm-3) in both ambients. It was found that for N2 annealing the diffusion in the low‐concentration region (≪1×1019 cm-3) is highly enhanced in the early annealing stage and gradually reduced with annealing time, and that this time‐dependent diffusion depends on the implantation energy and annealing temperature. Arsenic diffusivities and related parameters were extracted by fitting numerically calculated profiles to SIMS data. The time‐dependent diffusion was attributed to the diffusion of interstitial arsenic and its reaction with the SiO2 network.