A process for preparing solutions of sulfonated arylphosphines comprising reacting arylphosphines with oleum, diluting the sulfonation mixture with water, extracting the diluted sulfonation mixture with a solution of a water-insoluble organic solvent, reacting the organic phase with a solution of an inorganic base in water, adjusting the resulting aqueous solutin of sulfonated arylphosphines to a pH of up to 3 by addition of an acid, and passing an inert gas stream through the aqueous solution comprising sulfonated arylphosphines until sulfur is no longer liberated from the aqueous solution of sulfonated arylphosphines.