NOVELTY - A process for forming ceramic composites of an oxygen-ion conducting ceramic (a) and an alkali metal-ion conducting ceramic (b). The process involves: (i) forming a shaped composite of a precursor ceramic of alpha alumina and/or gallium oxide and (a) and (ii) subjecting the composite to a vapour containing the metal oxide of an alkali metal.    USE - The ceramics are used as electrolytes in sodium sulphur batteries, alkali metal thermoelectric converters (AMTEC) and alkali metal sensors.    ADVANTAGE - The materials are formed without sintering avoiding increased grain size and grain boundary liquid phase. Processing time is shortened.    DETAILED DESCRIPTION - Step (ii) is conducted at a temperature to convert the precursor to (b) in the form of a beta- or beta''-oxide of aluminium or gallium. The composite should be a continuous matrix. INDEPENDENT CLAIMS are also included for:    (I) a process for forming ceramic composites of (a) and an alkali metal-beta or beta''-alumina; the process involves forming a shaped composite of alpha-alumina and (a) and subjecting the to a vapour containing the metal oxide of an alkali metal to form alkali metal-beta or beta''-alumina;    (II) a process for forming ceramic composites of an oxygen-ion conducting ceramic and an alkali metal beta- of beta''-gallate;    (III) a ceramic composite comprising a continuous phase of an alkali metal ion conductor and a continuous phase of a ceramic oxygen ion conductor;    (IV) a ceramic composite comprising a continuous phase comprising alkali metal beta or beta''-gallate and a continuous phase of a ceramic oxygen ion conductor; and    (V) a ceramic composite comprising a continuous phase of a mixture of alkali-metal beta or beta''-alumina and alkali metal beta or beta''-gallate and a continuous phase of a ceramic oxygen-ion conductor.    DESCRIPTION OF DRAWING(S) - The drawing shows an alpha-alumina and oxygen-ion conductor showing the conversion process of alpha-alumina to sodium beta-alumina.    alpha alumina phase (103)    an oxygen-ion conductor or zirconia phase (103)    sodium beta alumina (105)    reaction front (107)