High resolution thermogravimetric analysis (HRTG) coupled with a gas evolution mass spectrometer was used to study the decomposition of peisleyite Na3Al16(SO4)(PO4)(10)(OH)(17).20OH(2)O. Peisleyite, a hydrated hydroxylated multianion mineral, shows thermal decomposition in five stages, indicative of a loss of water, hydroxyl units and sulphate. Thermal analysis shows the loss of water and hydroxyl units occurs in three distinct stages; at 36, 105 and 162 degreesC. The high resolution thermogravimetry was complimented with hot stage Raman spectroscopy, which follows the dehydration of peisleyite. This dehydration can be followed by the decrease in intensity of the OH stretching vibrations which occur between 4000 and 2500 cm(-1) and by the shift in the sulphate and phosphate peaks which occur due to changes in the mineral structure. (C) 2004 Elsevier B.V. All rights reserved.