Mg-based lamellar materials were prepared by high-energy milling. The XRD, SEM, TG and BET techniques were used to characterize the materials. Their hydrolysis properties in seawater were studied by measuring the hydrogen volume produced and by calculating the rate of hydrogen evolution. The solid hydrolysis products were analyzed by SEM and XRD. The experimental results showed that the milled samples were substantially more reactive than the unmilled Mg powder in air according to initial oxidization temperature. In the hydrolysis process, the unmilled Mg powder barely reacted with seawater. However, the milled Mg powder showed noticeable reactivity in seawater, generating 906 mL/g of hydrogen in 10 min, an evolution of 97.1% of the theoretical yield. The Mg-based hydroreactive materials including Co or Ni were more reactive in seawater than milled Mg powder due to the creation of a micro-galvanic cell. The Mg/Co sample produced 575 mL/g of hydrogen in the first minute. Mg(OH)(2) was the main reaction product of magnesium with seawater. It accumulated in a three-dimensional form composed of nanosized Mg(OH)(2) chip. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.