Five magnesium substituted aluminophosphate molecular sieves (denoted as MgAPO-5 (a-e), AFI topology) with different magnesium contents were synthesized hydrothermally and characterized by XRD, ICP-AES, IR, and NH3-TPD techniques. The characterization data show that the isomorphous substitution of magnesium for aluminum in the pure AFI framework results in MgAPO-5 (a-e) with weak and strong acid sites and that the number of strong acid sites linearly increases with increasing magnesium content (correlation coefficient: 0.9960). n-Hexane catalytic cracking over MgAPO-5 (a-e) was investigated at low conversions (a parts per thousand currency sign5%) in a microflow fixed bed reactor. Under controlled conditions, the reaction was first order in both the n-hexane concentration and the magnesium content. The analysis of the kinetic parameters and the product selectivities suggests that the cracking transformation is effected by strong Mg(II)-related acid sites rather than weak ones and predominantly undergoes the monomolecular protolytic mechanism, which is consistently explained by a proposed kinetic model.