The effect of metal ions in ash on enzymatic hydrolysis of crop straw has not attracted sufficient attention. In present study, rice straw with high ash content was taken as an example to study the effect. It was found that the common cations of ash (K(+), Mg(2+), Ca(2+), Al(3+), Mn(2+), Fe(3+), Cu(2+), and Zn(2+)) all showed inhibitive effects on cellulase at different levels, except for the stimulative effects of Ca(2+) and Mg(2+) on beta-glucosidase. Interestingly, washing treatment, combined with steam explosion, could effectively remove the ash cations of rice straw. The kinetic study of enzymatic hydrolysis indicated that, compared with the unwashed steam-exploded rice straw (SERS), the maximal hydrolysis velocity of washed SERS was increased by 12.5%. Therefore, the removal of ash by washing could facilitate the enzymatic hydrolysis of SERS. (C) 2010 Elsevier Ltd. All rights reserved.