A new robust Bi-carboxylate metal-organic framework (MOF) IEF-2 was hydrothermally prepared, acting as a proton carrier (σ∼ 1.1 × 10-4 S cm-1). Such proton conductivity is among the highest reported so far for purely 3D carboxylate-based MOFs, being explained by the existence of a 1D hydrogen-bond network, as suggested by structural analysis and theoretical studies.