A process for preparing phosphine oxides by reacting iminophosphoranes with phosphorus oxytrichloride by which highly purified phosphine oxides can be obtained industrially in a higher yield. Specifically, phosphine oxides are prepared in such a manner that iminophosphoranes are reacted with phosphorus oxytrichloride using an aprotic organic solvent with permittivity 2.2 or more at 20° C. as a solvent under special reaction conditions to give a liquid reaction product containing phosphine oxides and aminophosphonium chlorides which are yielded as a by-product at the same time as the phosphine oxides, and the above described chlorides are removed from the above described liquid reaction product by a solid-liquid separation process, and the solution having been subjected to the solid-liquid separation process is washed with water.