Uranium was effectively extracted from a synthetic aqueous monocalcium phosphate/phosphoric acid mixture (MCP/H3PO4) by tri-n-octylamine dissolved in different solvents and the presence of low concentrations of some counteranions (oxalate, citrate, chloroacetate) in the aqueous phase. The extraction efficiency was markedly enhanced by the salting out action of the concentration of calcium ions in the MCP/H3PO4 solution.