Nd tungstates and molybdates are promising materials for hydrogen separation membranes due to their high protonic conductivity. This work aims at elucidating the structural, textural and oxygen transport features of Nd_5.5WO_11.25−δ, Nd_5.5W_0.5Mo_0.5O_11.25−δ and (Nd_5/6La_1/6)_5.5WO_11.25−δ and their composites with Ni_0.5Cu_0.5O synthesized by mechanical activation. The oxide materials obtained were distorted double fluorites but their composites with Ni_0.5Cu_0.5O possess a complex phase composition. Extended defects such as grain boundaries, stacking faults and surface steps/terraces were observed in TEM images, which allow fast diffusion transport along grain boundaries (D* ∼ 10⁻⁶ cm² s⁻¹ at 700 °C) and slower diffusion within grains’ bulk (D* ∼ 10⁻¹¹, 10⁻¹² and 10⁻¹³ cm² s⁻¹ at 700 °C for the rather fast, “middle” and slow channels of bulk diffusion) (2D diffusion). The model gives the best description of experimental data obtained by the isotope exchange of oxygen with C¹⁸O₂ in a flow reactor. For composites with Ni_0.5Cu_0.5O, a significant decrease in oxygen diffusivity was shown. The reduction and subsequent reoxidation of composites resulted in an increase in oxygen mobility probably due to the partial unblocking of oxygen diffusion corresponding to the Ln tungstates/molybdates. Fine oxygen transport features allow us to increase the hydrogen yield of hydrogen separation membranes due to the proton transport mechanisms involving oxide anions and the water splitting reaction. Hence, the features of Nd tungstates and their composites with nickel(II)–copper(II) oxide studied demonstrated their high potential for use in catalytic reactors based on hydrogen separation membranes.