Secondary ion mass spectrometry was used to profile the diffusion of oxygen in polycrystalline β-cristobalite and vitreous SiO2. The tracer concentration profiles of cristobalite are consistent with a model based on two mechanisms: bulk and short-circuit diffusion. The profiles of partially crystallized samples containing vitreous SiO2 and β-cristobalite were fitted using the sum of two complementary error functions and taking account of some interstitial-network exchange. The bulk oxygen diffusivity, in the temperature range 1240-1500 °C, is about five times greater for vitreous silica than for β-cristobalite.