The search for new materials with pronounced proton or oxygen-ion conductivities is of great importance for the development of solid state ionic and electrochemistry fields. Here, we studied the structure, phase transitions, and ionic (oxygen-ion and proton) conductivity of the pure and Nd containing γ-La6W2O15-based composites and pseudorhombohedral La14-xNdxW4O33 (x = 12, 14) solid solutions. The proton conductor La14W4O33 (5 × 10−5 S/cm at 600 °C) was found to be a two-phase material consisting of an anion-deficient La10W2O21 fluorite-related phase and the γ-La6W2O15 orthorhombic phase. The phase content of the pure La10W2O21 cubic phase was ∼18 wt% for the γ-La6W2O15-based composite. A high degree of Nd content in γ-La6W2O15-based composite leads to formation of solid solutions based on a pseudorhombohedral phase in La14-xNdxW4O33 with x = 12 and 14. The Nd -containing γ-La6W2O15-based composites exhibited proton conductivity, which gradually decreased with increasing Nd content, whereas La14-xNdxW4O33 (x = 12, 14) pseudorhombohedral solid solutions were identified as oxygen-ion conductors. Nd14W4O33 has the oxygen -ion conductivity of ∼4 × 10−4 S/cm at 700 °C (1.0 × 10−3 S/cm at 900 °C). In contrast to the γ-La6W2O15 phase, the γ-La6W2O15-based composite undergoes only a single reversible phase transition at around 910 °C, which can, however, initiate cracks in ceramics. According to DSC and SEM data, the phase transition near 910 °C can be suppressed by introducing Nd into the γ-La6W2O15-based composites. The cracking process is enhanced by evaporation of tungsten oxide at T ≥ 1450 °C.