Abstract Composite electrolytes of the lithium-ion-conducting ceramic Li 1.3Al 0.3Ti 1.7(PO 4) 3 and polyetherurethane/lithium triflate polymer electrolyte have been prepared. Microscopy has shown that adhesion between the ceramic and polymer phases is poor, with gaps up to 1 μm at the interface. When dry, the composites are no more conductive than the pure polymer electrolyte. Exposing the samples to the vapour of solvents such as DMF, acetonitrile or water produces a significant increase in conductivity, over and beyond simple plasticization of the polymer. Pretreating the ceramic with a compatibilizing agent improves adhesion at the interface with the polymer, but decreases overall conductivity in the case investigated.