Metal ions can be removed from wood chips by acidic leaching. The rate of kraft delignification may be enhanced by such a chip pre-treatment, but whether this effect is present or not, is dependent on wood species and cooking liquor composition. Acidic chip leaching of birch (Betula pendula) and aspen (Populus tremula) was shown to give a large increase in the rate of delignification when carbonate-free white liquor, prepared in the laboratory, was used. Removal of calcium ions from the wood chips during the acidic leaching was shown to be the principal reason for the increase in the delignification rate. In trials where the cook was not preceded by acidic leaching, the addition of carbonate to the white liquor eliminated the retarding effect of calcium. The addition of the chelating agent DTPA (diethylenetriaminepentaacetic acid) also counteracted the calcium-effect. Since carbonate forms solid calcium carbonate precipitates and DTPA forms Ca-DTPA complexes, it can be concluded that non-chelated calcium ions in the cooking liquor decrease the rate of lignin solubilisation. The delignification rate of eucalyptus (Eucalyptus globulus) was not significantly affected by calcium-removal or by the addition of carbonate or DTPA to the white liquor. A tentative explanation for this could be that substances released from eucalyptus during cooking form complexes with calcium ions and may thereby prevent the calcium-effect on delignification. This explanation is supported by results from trials where birch and eucalyptus chips were cooked together.