From the outset Of the study of MOFs as proton conductors, both conductivity and hydrolytic robustness of the materials have needed to be improved. Here, we report a layered magnesium carboxyphosphonate framework, PCMOF10, that shows an extremely high proton conductivity value of 3.55 X 10(-2) S.cm(-1) at 70 degrees C and 95% RH. Moreover, PCMOP10 is water stable owing to strong Mg phosphonate bonding. The 2,5-dicarboxy-1,4-benzenediphosphohic acid (H6L) linker anchors a robust backbone and has hydrogen phosphonate groups that interact with the lattice water to form an efficient proton transfer pathway.