Recent neutron-diffraction experiments revealed the presence of hydroxonium ions in the hydrated HSAPO-34, an aluminophosphate type zeolitic material, structurally isotypic with chabazite. Ab initio molecular dynamics (AIMD) simulations were used to decide whether the proton transfer from a Bronsted acid site to a single water molecule is possible in this material, or the simultaneous presence of two water molecules in the zeolite cage-is necessary to realize such a transfer. The molecular dynamics calculations support the view that while the intrinsic acidity of the Bronsted site in HSAPO-34 is insufficient to protonate an isolated water molecule, the basicity of a hydrogen-bonded water dimer is high enough to act as proton acceptor at the acid site.