The hydrogen storage performance and electrochemical properties of Zr(1-x)xTi(x)(Mn0.2V0.2Ni0.6)(1.8) (X = 0.0, 0.2, 0.4, 0.6) alloys are investigated. All of these alloys have mainly a C14-type Laves phase: structure according to X-ray diffraction analysis. As the mole fraction of Ti in the alloy increases, the reversible hydrogen storage capacity decreases, while the equilibrium hydrogen pressure increases. Furthermore, the discharge capacity shows a maximum and the rate capability is increased, but the cycling durability is rapidly degraded with increasing Ti content in the alloy. The analysis of surface composition shows that the rapid degradation of Ti-substituted Zr-based alloy electrodes is due to the growth of;ln oxygen penetration layer. After comparing the radii of atoms and ions in the electrolyte, it is clear that the electrode surface becomes more porous. which is the source of growth of the oxygen penetration layer and causes accelerating the dissolution of alloy constituting elements with increasing Ti content. Consequently, the rapid degradation (fast growth of the oxygen-penetrated layer) with increasing Ti substitution in Zr-based alloy is ascribed to the formation of a porous surface oxide through which the oxygen atom and hydroxyl ion, with relatively large radii can easily transport into the alloy surface. (C) 2000 The Electrochemical Society. S0013-4651(99)11-0-18-6. All rights reserved.