Abstract: A novel process for producing a phosphoric monoester which comprises reacting per 1 mole of a linear or branched, saturated or non-saturated aliphatic alcohol having from 8 to 36 carbon atoms, from 2 to 10 moles of a polyphosphoric acid at 100 to 116% concetration converted as orthophosphoric acid in (a) one or more solvents selected from the group consisting of linear or branched saturated aliphatic hydrocarbons having from 4 to 8 carbon atoms and saturated cycloaliphatic hydrocarbons having from 5 to 7 carbon atoms, then adding (b) a lower alcohol having from 1 to 4 carbon atoms and water to the reaction mixture, transferring the orthophosphoric acid to the lower alcohol-aqueous phase to separate it from the phosphoric monoester, and concentrating the thus recovered orthophosphoric acid for reuse. The process reaction proceeds smoothly, separation and purification can be carried out readily and very effectively and the recovered phosphoric acid can be re-used.