Abstract Chromium(III) and iron (III) ions exchanged both into the pillars and into the micropore structure of pillared clays are compared during thermal treatments from 120°–480°C for various times and in different atmospheres (air and nitrogen). Thermal gravimetric analysis (TGA) is used to monitor the %H 2 O lost from 20°–720°C and X-ray diffraction (XRD) is used to elucidate structural information. Both untreated and heat-treated clays are then examined for resistance to acid-attack using solutions of HCl having a concentration range of 1.25 × 10 −4 N to 1N. Transition-metal leaching during acid treatment is monitored via atomic absorption (AA), and found to increase as the concentration of HCl increases. The % loadings of transition metal (TM) is seen to be an important factor with respect to stability of the clays. Since iron-containing materials in most cases contain higher concentrations of TM than chromium-containing clays, these materials have undergone more decomposition of the clay lattice and are less stable to acid attack. The exception is Fe + ACH-BL where iron(III) ions have been incorporated into the pillars of an alumina pillared clay. When this material is heat-treated it becomes more resistant to 0.05N HCl than ACH-BL without TM ions.