The reasons why the addition of some soluble phosphates to magnesium oxychloride cement (MOC) can greatly improve its water resistance are discussed. XRD analysis and the determination of the strength retention coefficients (a judging index for the water resistance) showed that the addition of a small quantity of soluble phosphates, such as H3PO4, NaH2PO4.2H2O, and NH4H2PO4, to MOC pastes does not influence the phases formed in the MOC pastes, but can greatly increase the strength retention coefficients of hardened MOC pastes, i.e., greatly improve the water resistance of MOC. The key components responsible for improving the water resistance could be the anions H2PO4-, HPO4(2-), and PO4(3-) yielded by the ionisation of these phosphates in the solution of MOC pastes. These anions can decrease the lowest concentration of Mg2+ ions in the solution required by the formation of 5Mg(OH)2.MgCl2.8H2O (5-phase) or 3Mg(OH)2.MgCl2.8H2O (3-phase) in the MOC pastes and increase the stability of these phases in water. Thus, when the hardened MOC pastes with a small quantity of the soluble phosphates are immersed in water, the 5-phase or 3-phase in them are not decomposed, leaving the strength of the hardened MOC pastes unchanged in water. 6 refs.