The thermal decomposition of a I ka ii meta I chlorides was studied on a series of colloidal a nd particulate aluminas with different crystalline phases. Decomposition was monitored by differential thermal and thermogravimetric analysis and the resulting phase development by X-ray diffraction. It was found that the presence of strongly bound lattice water in colloidal gel bohemite alumina caused significantly lower decomposition temperatures and increased reactivity towards compound formation. This has been attributed to the simultaneous loss of water via a hydrolysis reaction with the meta I chloride, a nd subsequent phase transformation of the pseudo-bohemite into gamma-alumina, forming the aluminate at a much reduced temperature.