Two mechanisms of doping Li₃NbO₄, which has an ordered, rock salt superstructure, have been established. In the “stoichiometric mechanism”, the overall cation-to-anion ratio is maintained at 1∶1 by means of the substitution 3Li⁺ + Nb⁵⁺ → 4Ni²⁺. In the “vacancy mechanism”, Li⁺ ion vacancies are created by means of the substitution 2Li⁺ → Ni²⁺. Solid solution ranges have been determined for both mechanisms and a partial phase diagram constructed for the stoichiometric join. On the vacancy join, the substitution mechanism has been confirmed by powder neutron diffraction; associated with lithium vacancy creation, a dramatic increase in Li⁺ ion conductivity occurs with increasing Ni content, reaching a value of 5 × 10⁻⁴ Ω⁻¹ cm⁻¹ at 300 °C for composition x = 0.1 in the formula Li_3−2xNi_xNbO₄. This is the first example of high Li⁺ ion conductivity in complex oxides with rock salt-related structures.