To develop proton-conducting materials with high hydrothermal and acid-base stability and to elucidate the proton-transport mechanism through visualized structural analysis, two new lanthanum phosphite-oxalates with 3D frameworks, designated as [La(HPO₃)(C₂O₄)_0.5(H₂O)₂] (La-1) and (C₆H₁₆N₂)(H₃O)[La₂(H₂PO₃)₃(C₂O₄)₃(H₂O)] (La-2) (C₆H₁₄N₂=cis-, 6-dimethylpiperazine), were prepared by hydrothermal and solvothermal conduction, respectively. La-1 was constructed with lanthanum phosphite 2D layers and C₂O₄^2- groups, whereas La-2 was constructed with lanthanum oxalate 2D layers and H₂PO₃^- groups. Alternating current (AC) impedance spectra indicate that the proton conductivities of both compounds could reach 10^-4 S•cm^-1 and remain highly durable at 75 ℃ and 98% of relative humidity (RH). Due to the abundance of H-bonds in La-2, the σ of La-2 was higher than that of La-1. La-1 exhibited excellent water and pH stability.