Abstract: The method for preparing a monoalkylphosphine of high purity comprises reacting phosphine and an alkene in the presence of an anhydrous alkanesulfonic acid as a catalyst in a solvent having a boiling point higher than that of the monoalkylphosphine produced. The resulting monoalkylphosphine is brought into contact with an alkali solution to remove the catalyst, which is a sulfur atom-containing compound, remaining in the solution. The reaction solution from which the remaining catalyst is removed is then brought into contact with alkali metal hydrides or alkaline earth metal hydrides to eliminate any moisture remaining in the reaction solution. The alkali metal hydride is preferably sodium hydride or lithium aluminum hydride, while the alkaline earth metal hydride is preferably calcium hydride.