2/1-mullite single crystal (001) plates with thicknesses between 0.9 and 1.9 mm were exposed for 1.5, 3, 6, and 12 h at 1670 degrees C to a slowly flowing (100 mL/min) water-rich gas mixture (O-2/H2O 80/20). Under the given experimental conditions, 2/1-mullite yielded significant amounts of structurally bound OH groups across the bulk and decomposition of the crystal surface on a micrometer scale. Decomposition products are (i) sodium-containing silicon-rich alumino silicate glass formed from melt and (ii) alpha-alumina, which crystallizes within melt cavities. The crystal plates that are free of any OH absorption before the corrosion experiments show a steep increase in OH absorption intensity up to 3 h of corrosion and a flattening toward longer times of exposure. The evaluation of OH intensity profiles implies an effective diffusion coefficient D-H in the range between 1.5 and 2.5 x 10(-7) cm(2)/s.