Two synthetic approaches to the preparation of mixed aluminium phosphite-phosphonate solids have been made. First, the hydrothermal reaction of aluminium hydroxide (gibbsite) with mixtures of phosphorous and methylphosphonic acids under conditions that give microporous aluminium methylphosphonate-β (AlMePO-β) when methylphosphonic acid alone is used, and secondly, the reaction of AlMePO-β with increasing amounts of molten phosphorous acid. Under hydrothermal conditions there is no evidence that AlMePO-β can be prepared with phosphite groups randomly replacing methylphosphonate groups. Rather, the products are dominated over the intermediate phosphite/phosphonate compositional range by a novel phase that is thought, on the basis of ²⁷ Al and ³¹ P MAS NMR and FTIR spectroscopies, to contain differing amounts of phosphite and methylphosphonate groups. Reaction of AlMePO-β with levels of molten phosphorous acid at 40% or more of the methylphosphonate content gives mixtures of AlMePO-β and a new phosphite phase, whereas reaction using lower amounts of the molten acid leaves AlMePO-β as the only X-ray visible phase. Extension of the melt method to the separate reaction of gibbsite with methylphosphonic and phosphorous acids yields, respectively, single crystals of a new aluminium methylphosphonate [Al(O ₃ PCH ₃ )(HO ₃ PCH ₃ )·H ₂ O] and a known aluminium phosphite [Al ₂ (O ₃ PH) ₃ ·4H ₂ O], the structure of which had only been solved from powder diffraction data. Single crystal diffractometry improved the accuracy with which the structural parameters of the phosphite are known and enabled structure solution of the new aluminium methylphosphonate [Pnma, a=19.075(6) Å, b=5.117(2) Å, c=8.439(2) Å], which is made up of layers that contain isolated, octahedrally coordinated aluminium linked by methylphosphonate groups.