In the hydrogen absorption reaction, the surface condition is a predominant factor. The surface oxide layer generally prevents the dissociative absorption. Recently we reported the effect of the pretreatment by LiOH, which enhanced the hydrogen absorption even after significant surface oxidations. In this study the surface of LaNi5 alloy treated with a LiOD solution was analyzed by TOF-SIMS. The reaction mechanism of the accelerated absorption by the alkaline treatment is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.