A lamellar material composed of alternating layers of magnesium phosphate sheets and n-hexadecylammonium (n-C16H33NH3+) ions was synthesized by the reaction between magnesium acetate tetrahydrate and phosphoric acid. The chemical formula of the composite material was determined to be (n-C16H33NH3+)Mg2+(HPO4(2-))(H2PO4-) by ICP, TG, and 31P MAS NMR measurements. The lamellar composite was obtained under basic conditions adjusted with tetramethylammonium (TMAOH) and tetraethylammonium hydroxides (TEAOH), while ammonium ions did not act as an alkali source useful for obtaining the composite material. The formation of crystalline byproducts such as hydrated newberyite (MgHPO4.3H2O) and ammonium-type struvite (MgNH4PO4. 6H2O) was suppressed by using the EtOH/H2O mixed solution and by adding TMAOH and TEAOH to adjust the synthetic condition, respectively. Including the data on calcium, barium and strontium phosphates, we can totally understand the formation of the mesostructured materials composed of ionic frameworks in the presence of n-alkylamines in detail. It is quite important to suppress the formation of crystalline alkaline earth metal phosphates and control the solubilities of reactants and products for synthesizing pure mesostructured materials with ionically crosslinked frameworks.