The substitution of silicon by aluminum at different tetrahedral sites of zeolite-beta in the presence of H+ and Na+ has been investigated by density functional calculations on pentameric model clusters. It is found that the relative substitution energies are within a narrow range, which is 1.4 kcal/mol in the presence of proton and 0.7 kcal/mol in the presence of Na+. For the protonated case, the Al substitution is the least favored at sites associated with two four-membered rings. The structure of the AlO4 unit is considerably distorted in the protonated clusters due to the formation of a strong OH band. The sodium ion has a less significant effect on the geometry of the AlO4 unit, since it forms much weaker bonds with two O atoms.