The effects of humidified atmosphere on oxygen surface exchange and diffusion in La2Mo2O9 have been investigated. After annealing samples in 020 vapour, the depth and line scan profiles of the OD- species showed the incorporation of hydroxyl groups on the surface and of diffusion into the bulk. The hydroxyl diffusion process appears to be different from that of oxygen diffusion, and might indicate the existence of ambipolar diffusion of oxide ions and hydroxyl species in La2Mo2O9. (C) 2010 Elsevier B.V. All rights reserved.