Summary: The high-temperature transformation of andalusite (Al₂SiO₅) into 3/2-mullite (₃A₂O₃2SiO₂) plus vitreous silica (SiO₂) has been studied within the temperature interval from 1300 to 1600 °C by means of X-ray powder and single-crystal techniques and by infrared spectroscopy. Results are interpreted in terms of a topotactic transformation in which (011) and (01¯1) planes of andalusite transform into (20¯1) and (201) planes of 3/2-mullite; in this way the a- and b-axes of the two phases are interchanged. From the structural and orientational relationship between the andalusite and 3/2-mullite latrices it is concluded that the aluminium-oxygen octahedral chains, running in both structures parallel to the crystallographic c-axis, are preserved during phase transformation.