Li+ ionic conductors are able to undergo topotactic exchange reaction in acidic medium. This reaction is illustrated, at room temperature, in the case of lithium lanthanum titanate powder (Li3xLa2/3-x square(1/3-2x))TiO3 (named LLTO). Thermogravimetric analysis, infrared spectroscopy, Li-7 NMR and H-1 NMR clearly showed that such an exchange reaction also occurs on the microstructured powder of LLTO at room temperature after several months in ambient atmosphere (relative humidity: 50%). The reaction with water and CO2 contained in air leads to the formation of partially protonated titanates and lithium carbonates at the surface of the grains. A mechanism of reaction is proposed: the titanate would be able to dissociate water on the grain surface and then exchange H+ for Li+ into the perovskite structure, and lithium hydroxide is then formed on the grain surface and reacts with CO2 contained in air to form Li2CO3. It will also be shown that this mechanism is reversible. This surface reaction is greatly enhanced by the use of nanomaterials and the increase of the specific surface area of the powder. It occurs rapidly after synthesis under ambient atmosphere. Therefore great care has to be taken in the storage of powders made of nanograins to avoid any chemical reaction with the surrounding environment. (C) 2010 Elsevier B.V. All rights reserved.