Substitution of B for Ti in Li1+x Ti2−x B x (PO4)3 introduces x mobile Li+ ions per formula unit in the solid-station range 0< x ≤0.2. The B2O3 is also an aid to sintering dense ceramics with more uniform particle size. With x =0.2, the room-temperature Li+-ion conductivity is σ Li =2.0×10−4 Scm−1 with a motional activation energy of 0.48eV. For x >0.2, an intergranular B2O3-rich second phase impedes intergranular Li+-ion transport.