The wetting of mullite (Al₂O₃ . SiO₂) by a Y₂O3-Al₂O₃-SiO₂ (YAS) glass and by a B2O3-SiO2 (borosilicate) glass was investigated in air as a function of temperature through a sessile drop technique. The wetting behavior was found to be strongly dependent on the glass composition and on the temperature. For the YAS glass, the contact angle showed a rapid decrease from 100⁰ to 20⁰ in the temperature range of 1400 to 1450 ⁰C followed by a more gradual decrease to a value of 10-15⁰ at approximately 1600 ⁰C. In the case of the borosilicate glass, the change in the contact angle with temperature was more uniform, with the value decreasing from 100⁰ to 45⁰ between 1300 and 1600 ⁰C. Microstructural observations of the interfacial region between the solidified sessile drop and the mullite substrate revealed significant corrosion of the interface and penetration into the mullite grain boundaries by the YAS glass. In the case of the borosilicate glass, corrosion was limited and the interface was clearly defined. The consequences of the data for the design of in situ toughened mullite are discussed.