The microscopic mechanisms of O-2 diffusion in compressively strained high-density silicon oxides are investigated based on first-principles total-energy calculations. It is found that, both in high-density alpha-quartz and in alpha-cristobalite, the calculated incorporation energies and energy barriers increase with increase of oxide density. Independent of the structure of oxides, the calculated activation energies increase with increasing density. Furth en-no re, the calculated activation volumes suggest that the oxidation retardation by the oxidation-induced strain is due to the retardation of O-2 diffusion in the high-density region, qualitatively consistent with experimental results. (c) 2005 Elsevier B.V. All fights reserved.