Aluminum formate and basic acetate gels obtained from commercial pseudoboehmite were synthesized in this work. Phase evolution after successive thermal treatments was studied on the raw material and both gels. In both carboxylates the influence of aging time on the composition, the crystalline structure and the phase transition temperatures was studied by X-ray diffraction (XRD) and differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Two different stages were observed in the thermal decomposition of both carboxylates: the first, corresponding to the decarboxilation and dehydration of the gels. was different for both cases because of the different type of carboxilic acids used; and the second, corresponding to the formation of transition aluminas, was the same whatever the precursor used. However, whenever the aging time was changed on any of the carboxylates, the mechanisms of the thermal transformations were affected. (C) 2003 Elsevier B.V. All rights reserved.