MAPSO-39 and MAPSO-43 have been synthesized, the latter material obtained as a pure phase in the absence of the usual cocrystallizing MAPSO-46 phase. Si-29 NMR data and chemical analysis support the incorporation of silicon into the framework of the two materials, though this is only to a limited extent. The different P(nMg) environments have been identified in the P-31 NMR spectra of both materials. For MAPSO-43, the relative populations of P(nMg) environments are consistent with the random distribution of Mg among the Al sites of the framework. Analysis of the possible ordered configurations of Mg in the framework is also not consistent with the observed NMR data.