Metal organic framework (MOF) materials are a nontraditional route to ion conductors, but their crystallinity can give insight into molecular-level transport mechanisms. However, some MOFs can be structurally compromised in humid environments. A new 3D metal-organic framework, PCMOF-5, is reported which conducts protons above 10(-3) S/cm at 60 degrees C and 98% relative humidity. The MOF contains free phosphonic acid groups, shows high humidity stability, and resists swelling in the presence of hydration. Channels filled with crystallo-graphically located water and acidic groups are also observed.