Summary: The temperature at which dehydroxylation occurs in amphiboles is thought to be largely dependent on the type of cation that occupies the M₃ and M₁ sites. Four amphibole types have been studied in which these sites are occupied, in various proportions, by Mg²⁺ or Fe²⁺. The temperature of dehydroxylation, in an inert atmosphere, is shown to rise with increasing Mg²⁺ content in these sites.