The relative strength for Bronsted acid sites of isomorphously substituted H-MeAlPO-5 molecular sieves (Me = Si, Ti, or Zr) is investigated using density functional theory employing periodic models. The results are consistent with the experimental observation and show that H-ZrAlPO-5 has the weakest Bronsted acid sites. Furthermore, the Bronsted acidity of H-MeAlPO-5 (Me = Si or Ti) is compared with that of H-MeAlPO-34 (Me = Si or Ti) [M. Elanany, K. Koyama, M. Kubo, P. Selvam, A. Miyamoto, Micropor. Mesopor. Mater. 71 (2004) 51]. (c) 2006 Elsevier B.V. All rights reserved.