Lithium orthosilicate, Li4SiO4, and its solid solutions are a new group of solids of high ionic conductivity. Either Li or Si may be at least partially replaced by several other di-, tri- or tetra-valent cations. Highest conductivities were found for solid solutions with Si partially replaced by Ti; typical values were 10−3 to 10−4 Ω−1 cm−1 at 300°C rising toc. l Ω−1 cm−1 by 700°C. The solid solutions are easy to prepare and are stable in air. Conductivities were measured on pressed cubes of powdered material using variable frequency a.c. methods. The conductivities of these Li4SiO4 solid solutions are compared with those of other known Li-conducting solids; Li4SiO4 solid solutions are probably the best all-round, polycrystalline Li-electrolytes yet found for temperatures abovec. 200°C.