The chemical and electronic nature of the various phosphorus environments in the microporous magnesium aluminophosphate MgAPO-20 has been investigated by P-31 magic-angle-turning NMR. Although the P(4Al) unit is highly symmetric, Mg incorporation into the lattice distorts this symmetry by altering the three P-31 shielding tensors unevenly and thus producing chemical shift anisotropy (CSA) patterns. The P(3Al, 1Mg) unit is close to axial symmetry whereas P(2Al, 2Mg) is highly asymmetric. The dramatic change in the electronic nature of phosphorus by magnesium substitution correlates with the number of magnesium atoms adjacent to phosphorus. These experimental findings not only reveal a wealth of information on the electronic nature of phosphorus in MgAPO-20 but also constitute strong evidence as to the magnesium incorporation into the AlPO-20 lattice. In like manner, this contribution may extend to a new strategy for the understanding of isomorphous substitutions in microporous materials.