Acid-activated clays (montmorillonites) have been cation-exchanged with aluminium chloride pentahydroxide dihydrate (aluminium chlorhydrate, ACH) solutions of different concentrations to yield expanded materials. Various Al:clay (a/c) ratios have been used. The effect of changing the degree of polymerisation of the pillaring Al species on both the properties and the chlorophyll adsorption capacity of the exchanged clays was investigated by varying the preparation method of the pillaring reagent so as to include ageing at high temperature. From powder XRD data it is shown that at least a 50% excess of the stoichiometric Al is necessary to obtain a material with an observable (001) reflection characteristic of an expanded (pillared) solid. The amount of alumina incorporated (Al_p) into the clay is critically dependent upon the Al:clay ratio and the method of preparation of both the pillaring solution and the precursor-pillared clay. Whereas no clear correlation exists between the interlayer spacing and Al_p, both the pore volume and the chlorophyll adsorption capacity of the materials are enhanced by exchange with highly polymeric cations. At low Al:clay values (<3.0 mmol g^–1 of clay; i.e. 0.03 mol dm^–3 ACH) the Al_p is dependent on the degree of polymerisation still present in such dilute pillaring solutions. At higher Al:clay ratios (12 mmol g^–1 of clay and above; i.e. 0.12 mol dm^–3 ACH) polymerisation is not a limiting factor and, therefore, the extent of hydrolysis of the polycations and their mobility determine the Al_p.