The diffusivity of oxygen in an aluminosilicate glass from 630° to 830°C is derived from optical transmission measurements, wherein the rate of bleaching of Ti3+color centers by the diffusing oxygen at temperature was measured. The finite‐thickness solution to the heat equation is used to interpret the data; the oxygen diffusivity on this temperature range can be represented byD=28.4 exp (−39.6±3.4 kcal/RT)cm1/sec. It is concluded that the oxygen diffuses interstitially as molecular oxygen, because the activation energy is smaller than the energy to split an Si&sngbnd;O bond: 50 kcal/mole. As a check on the technique and results, penetration measurements were made on a moving color boundary, with identical results.