Australia is the largest exporter of iron ore in the world. The high Al content in Australian hematite/goethite ores is detrimental to blast furnace and sinter plant operations. Of the Al containing minerals in iron ore, kaolinite is a common gangue mineral frequently found in iron ore deposits. In iron ore flotation pulp, various ions exist and may interfere with the flotation performance when their concentration is high enough. In this work, the role of a water structure-making ion, i.e. Na + , and a water structure-breaking ion, i.e. K + , in the flotation of kaolinite under reverse cationic flotation conditions, the most widely used flotation route of iron ore in the world, was studied in a series of laboratory batch flotation tests. It was found that K + , a water structure breaker, can better reduce the zeta potential of kaolinite and thus causes higher flotation recovery of the clay mineral, in comparison to Na + , a water structure maker. The different effects of the alkali metal cations on kaolinite flotation are attributed to the different aggregation degree of kaolinite particles in the presence of these cations.