This work investigates the effect of hydrogen on the microstructure of phosphate-bonded high-alumina refractories applied in the working lining of direct reduction reactors (DRR). These types of refractories are mainly composed of alumina, andalusite, mullite, and a glassy phase. Samples were exposed to a 100% hydrogen atmosphere at 900°C for a period of 16 and 48 h. The as-received refractory microstructure was composed of a matrix of needle-shaped mullite grains embedded in a minor glassy phase and agglomerates of corundum and andalusite. Under the exposure conditions, mullite transformed to corundum, which is the major microstructural transformation representing a considerable alteration of the refractory matrix and results in a gradual increase in the glassy-phase content. Besides, a low mass loss of about 0.075% has been observed, although the hydrogen caused some effects on the microstructure such as reduction of the mullite. Moreover, energy-dispersive X-ray spectroscopy of the postexposure material showed higher silicon content in the glassy phase, due to the reoxidation of silicon monoxide. Thermodynamic calculations confirmed the reduction of mullite and the formation of corundum under hydrogen.