A local density functional study of the structure and energetics of offretite, with a Si4+ ion substituted by (Al3+, M(+)) (M = Na, K), is presented. The calculations have been performed within the framework of the first principles molecular dynamics method using a periodically repeated unit cell with 55 atoms. It is found that the preferred site for K+ cations lies inside the cancrinite cage, in agreement with experiment. This site is related to Al substituting a Si atom at a T-1 site, which is one of the two inequivalent tetrahedral sites of offretite, and belongs to the hexagonal prism. Site selectivity is quite pronounced in the case of K+, as the large size of this cation gives rise to important steric effects;in the offretite framework. For Na+, instead, the lowest energy site is located in the channel, near a window of the cancrinite cage, and corresponds to an Al in T2. For this cation the spread in the relative substitution energies of the different binding sites is very small, the four sites of lower energies being in a range of similar to 2.5 kcal/mol.