We report a solid state nuclear magnetic resonance study of Na-23, Al-27, and P-31 in two crystalline nitridophosphate phases, Na3AlP3O9N and Na2Mg2P3O9N, including two-dimensional multiple-quantum magic angle spinning (MQMAS) experiments on Na-23 to separate overlapping lines. The previously described single-crystal structure of Na3AlP3O9N gives crystallographic examples of Al(OP)(6) and P(O[Al,Na])(2)(ONa)(N[P,Na]) environments and three different environments of sodium : two Na(O)(6) and one Na(O)(6)(N). From these observations we characterize the modification of the local environment of phosphorus and show that Mg only substitutes Na in the Na2 site of the Na2Mg2P3O9N structure.