Lithium aluminium titanium phosphate (LATP) belongs to the class of materials exhibiting very high Li⁺ ionic conductivities. While many impedance studies can be found in literature that report on overall ion conductivities, a discrimination of bulk and grain boundary electrical responses via conductivity spectroscopy has rarely been reported so far. Here, we took advantage of impedance measurements that were carried out at low temperatures to separate bulk contributions from the grain boundary response. It turned out that bulk ion conductivity is by at least three orders of magnitude higher than ion transport across the grain boundary regions. At temperatures well below ambient long-range Li ion dynamics is governed by activation energies ranging from 0.26 to 0.29 eV, depending on the sintering conditions. As an example, at temperatures as low as 173 K, the bulk ion conductivity, measured in N₂ inert gas atmosphere, is in the order of 8.1 x 10⁻⁶ S cm⁻¹. Extrapolating this value to room temperature yields ca. 3.4 x 10⁻³ S cm⁻¹ at 293 K. Interestingly, exposing the dense impedance pellets to air atmosphere over a long period of time causes a significant decrease of bulk ion transport. The effect can be reversed if the phosphate is calcined at elevated temperatures again.