The H+/Li+ ion exchange reaction of lithium lanthanum titanate (LLTO) nanopowders and films with water and CO2 contained in air, leads to the formation of a partially protonated titanates and carbonates at the surface of the oxide. A mechanism of surface reaction is proposed: the titanate undergoes reaction with water, then a topotactic exchange reaction of H+ for Li+ takes place into the perovslcite structure, lithium hydroxide (or lanthanum oxyhydroxide) is then formed on the grain surface and reacts with CO2 contained in air to form Li2CO3 (or lanthanum carbonate). This surface reaction occurs rapidly after synthesis under ambient atmosphere and is greatly enhanced by the use of nanomaterials or films. It is reversible if the material is heat-treated around 300-400 degrees C. Rutherford Backscattering Spectroscopy (RBS), Nuclear Reaction Analysis (RNA) and Elastic Recoil Detection Analysis (ERDA) are used to determine the composition of the films after storage in ambient air and to confirm the occurrence of an exchange reaction at the surface of the oxide stored under ambient atmosphere. (C) 2013 Elsevier B.V. All rights reserved.