Activation performance of TiMn1.25Cr0.25 hydrogen storage alloy with various particle sizes was investigated. For the samples that have not been exposed to air, the smaller is the particle size, the easier the activation becomes. However, after exposure to air for 1 day before activation, the activation of the alloys becomes more difficult with decreasing particle size. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) results show that there is a 6-8 nm thickness oxide layer formation at the top surface of air-exposed alloys, which plays an important role in effecting the activation performance of TiMn1.25Cr0.25 alloy. As indicated by particles analyzer, the activation process of the alloy is closely related with fragmentation of alloy particles. (C) 2003 Elsevier B.V. All rights reserved.