The layered Dion-Jacobson phase perovskites HLaTa₂O₇∙xH₂O, HLaNb₂O₇∙0.5H₂O and structural variants of HLaNb₂O₇∙0.5H₂O were prepd. and their a.c. and d.c. conductivities were measured as a function of temp. and atm. The layered structure in HLaNb₂O₇∙0.5H₂O was modified by pillaring, exfoliation-restacking, and further annealing. Exfoliation followed by restacking markedly increased the ionic cond., either by creation of mesopores for hydration or by enhanced contact between layers, but pillaring had little effect. HLaNb₂O₇∙0.5H₂O was prone to redn. under hydrogen, yielding a mixed ionic-electronic conductor. In-plane cond. measurements of an oriented film of HLaNb₂O₇∙0.5H₂O showed enhanced ionic cond. attributed to the anisotropic cond. of the sheets, but redn. under a hydrogen atm. was apparent. The redn. could be suppressed by adding water vapor to the ambient atm. or by replacing the B-site niobium with tantalum, which is less easily reduced. [on SciFinder(R)]