A method for the measurement of the diffusion of oxidant through a growing SiO2 film is presented. The procedure is based on so-called lag-time diffu sion methods in which the time to achieve steady-state oxidation is measured using in situ ellipsometry. Two different modes of oxidant transport were ob served over the range of temperatures investigated (600°-1000°C). At tempera tures of 900°C and below, no lag-time was observed, and steady-state oxida tion was seen at the outset of oxidation. At 1000°C, a lag-time was measured which yielded a value for the diffusion constant, D = 2.3 ˟ 10-13 cm2/sec for dry O2, and this value increased to 2.4 ˟ 10-12 cm2/sec for 1000 ppm H2 in O2. This study provides clear evidence for different dominant modes of oxidation at higher and lower oxidation temperatures. © 1982, The Electrochemical Society, Inc. All rights reserved.