The first aluminum methylphosphonate with a layered structure, designated AlMepO-zeta, giving an ideal composition of Al(OH)(O3PCH3). H2O was synthesized with a hydrothermal procedure. AlMepO-zeta is monoclinic, space group P2(1)/c, with a = 7.8676(7), b = 7.0744(13), c = 9.4789(7), beta = 106.599(6)degrees. The framework in a sheet containing unidimensional [-Al-(OH)-Al-] chain is topologically the same as VO(O3PC6H5). H2O. Methyl groups protrude into the interlamellar space. TG-DTA and MAS-NMR results revealed the decomposition mechanism.