In hot-compressed water (HCW), water-insoluble Mg and Ca become leachable from mallee wood, leaf, or bark. The extent of leaching of Mg and Ca varies from component to component in order of leaf > wood > bark for Mg and wood > leaf > bark for Ca. Such differences are due to two reasons: differences in the occurrence forms of Mg and Ca and differences in hemicellulose decomposition for different biomass components. For example, most water-insoluble Mg in wood and bark is ion-exchangeable, but only part of water-insoluble Mg in leaf is ion-exchangeable, with the rest being acid-soluble but not ion-exchangeable. Most water-insoluble Ca in wood is ion-exchangeable, but those in leaf and bark are acid-soluble but not ion-exchangeable. The results show that water-insoluble Mg or Ca in the ion-exchangeable form can be rapidly leached in HCW, but the leaching Of water-insoluble Mg or Ca in the acid-soluble but not ion-exchangeable form is considerably slower as a result of the poor solubility of these species in HCW. Only the leaching of ion-exchangeable Mg or Ca correlates well with arabinose recovery during biomass decomposition in HCW because these ion-exchangeable species are associated with acid functional groups on hemicellulose. Therefore, the leaching of water-insoluble Mg in all three components and water-insoluble Ca in wood depends strongly upon hemicellulose decomposition, resulting in the extent of sugar recovery during the hemicellulose decomposition (especially arabinan) following similar orders. Further analysis indicates that the leaching of water-insoluble Mg and Ca and the recovery of arabinose from all three biomass components follow first-order kinetics, except for the leaching of water-insoluble Ca in leaf that follows a two-step process consisting of an initial fast step for the leaching of Ca in the ion-exchangeable form, followed by a slow step for the leaching of Ca in the acid-soluble but not ion-exchangeable form.