The synthesis, structural characterization, luminescence properties, and proton conduction performance of a new family of isostructural cationic 2D layered compounds are reported. These have the general formula [Ln(H4NMP)(H2O)(2)]Cl center dot 2H(2)O [Ln = La3+, Pr3+, Sm3+, Eu3+, Gd3+, Tb3+, Dy3+, Ho3+, H6NMP = nitrilotris(methylphosphonic acid)], and contain Cl- as the counterion. In the case of Ce3+, a ID derivative, [Ce-2(H3NMP)(2)(H2O)(4)]center dot 4.5H(2)O, isostructural with the known lanthanum compound has been isolated by simply crystallization at room temperature. The octa-coordinated environment of Ln(3+) in 2D compounds is composed by six oxygen atoms from three different ligands and two oxygens from each bound water. Two of the three phosphonate groups act as both chelating and bridging linkers, while the third phosphonate group acts solely as a bridging moiety. The materials are stable at low relative humidity at less at 170 degrees C. However, at high relative humidity transform to other chloride-free phases, including the 1D structure. The proton conductivity of the ID materials varies in a wide range, the highest values corresponding to the La derivative (sigma approximate to 2 x 10(-3) S.cm(-1) at RH 95% and 80 degrees C). A lower proton conductivity, 3 x 10(-4) S.cm(-1), was measured for [Gd(H4NMP)(H2O)(2)]Cl center dot 2H(2)O at 80 degrees C, which remains stable under the work conditions used. Absorption and luminescence spectra were recorded for selected [Ln(H4NMP)(H2O)(2)]Cl center dot 2H(2)O compounds. In all of them, the observed transitions are attributed solely to f-f transitions of the lanthanide ions present, as the H4NMP2- organic group has no measurable absorption or luminescence properties.