We have studied by volumetric adsorption isotherm measurements the sorption properties of different AlPO4-5 or SAPO-5 samples, for sorbate species characterized by a large range of molecular size : D-2, Kr, Ar, C2D2, CO2, CH4, CF4, and C(CH3)(4). The highest sorption capacities, Q(max) greatly depend on the sorbate molecular size. A difference of 1 order of magnitude is observed between hydrogen and neopentane. In addition, we have found that the sorption capacities of the four analyzed AlPO4-5 or SAPO-5 samples vary by almost a factor of 2. This result clearly shows that, as a consequence of the one-dimensional character of the AFI micropore network, Q(max) is very sensitive to small amounts of crystalline defects or impurities. For methane, Q(max) is equal to 6 mol/uc, a quantity that is not compatible with the commonly accepted value of the internal diameter of the AlPO4-5 micropore diameter, (M) = 7.3 Angstrom. Moreover, an interesting phase transition phenomenon has been observed during the sorption of krypton and methane, a result that can be the consequence of the good parametric agreement between such sorbate species and the AlPO4-5 inner surface.