The literature data on state diagram of Al2O3-SiO2 system are analyzed. The phase composition of this system and the decomposition of mullite with the reversible transition to sillimanite are considered. The latter reaction is important for the theory of metallurgical processes. The congruent and incongruent state of mullite, liquation in solid phase (segregation of glass mixture), and structure parameters of mullite and sillimanite are examined.