A series of MgAPO and MgAPSO molecular sieves with a structure of AlPO4-5 or chabazite were synthesized with Mg/Al mole ratios ranging from 0.053 to 0.43 and the substitution of magnesium was thoroughly investigated by P-31, Al-27 magic-angle spinning (MAS) nuclear magnetic resonance (NMR) in combination with X-ray diffraction (XRD). In the case of MgAPO, pure MgAPO-5 can be synthesized only at a low Mg mole content (y) of 0.1 (corresponding to Mg/Al = 0.053) and the MgAPO-5 product will be associated with a certain amount of MgAPO-34 when y exceeds this value and no pure MgAPO-34 can be formed even at y = 0.6 (corresponding to Mg/Al = 0.43). For the MgAPSOs, a mixture of MgAPSO-5 and MgAPSO-34 is present even at y = 0.1 and pure MgAPSO-34 is formed when y exceeds 0.4 (Mg/Al = 0.25). The preferred formation of MgAPO-34 (or MgAPSO-34) rather than MgAPO-5 (or MgAPSO-5) with increasing the Mg content may partially result from the difference in the bond lengths of Mg-O and Al-O (P-O and Si-O) and partially from the different structural nature of AlPO4-5 and chabazite. NMR results indicate that magnesium ions can substitute for aluminum and four local structural units : P(4Al), P(3Al,1Mg), P(2Al,2Mg), and P(1Al,3Mg) can be formed in the frameworks of both MgAPO and MgAPSO molecular sieves. It is probably the increase in the amounts of 2Mg and 3Mg substitutions that favors the formation of MgAPO-34 (or MgAPSO-34). : P(4Al), P(3Al,1Mg), P(2Al,2Mg), and P(1Al,3Mg) can be formed in the frameworks of both MgAPO and MgAPSO molecular sieves. It is probably the increase in the amounts of 2Mg and 3Mg substitutions that favors the formation of MgAPO-34 (or MgAPSO-34).