The theoretical analysis of the intrinsic ionization constant (K-a2(int)) of Fe-Al-Mg hydrotalcite-like compounds (HTlc) possessing permanent a2 charges was first performed using the double extrapolation method proposed by James et al. The theoretical permanent charge density (sigma (p),T) of the HTlc sample was calculated from the crystal structure of HTlc, and the influence of sigma(p),T on the K-a2(int) was also examined. From the a2 experimental results, these conclusions can be obtained: the zero point of the charge (pHzpc) of Fe-Al-Mg HTlc increases with decreased Fe3+ content and increases with increased Mg2+ in the HTlc. The pK(a2)(int) of Fe-Al-Mg HTlc also increases with decreased Fe3+ and increased Mg2+ content in the sample; furthermore, the pK(a2)(int) of Fe-Al-Mg HTlc increases with decreased sigma (p,T). (C) 2003 Elsevier Science (USA). All rights reserved.