The activation performance of TiMn1.25Cr0.25 alloy without and with exposure to air prior to activation was compared at P = 1 MPa and T = 293 K and the possible underlying mechanism was discussed. The alloy, which has not been exposed to air before activation, can be activated with ease under moderate activation conditions. However, with increasing duration of exposure, its full activation became more difficult. X-ray photoelectron spectroscopy and Auger electron spectroscopy results show about 4-7 nm thick oxide layer on the surface of the air-exposed samples, which implies that formation of surface oxide layer may play an important role in controlling activation. As indicated by scanning electron microcopy, the activation process of the alloy is closely related with fragmentation of alloy particles. According to X-ray diffraction results, activation leads to expansion of the crystal lattice of the alloy and the expanded structure would not be restored even after hydrogen desorption, which benefits the decrease in activation energy for hydrogen absorption and desorption and makes the process easy. (C) 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.