To provide a method, capable of selectively determination a sulfonic acid type anionic surfactant of about 1 ppm in a solution such as an electrolytic solution or a plating solution, containing several - several ten g/l of heavy metal ions and a very small amount of the sulfonic acid type anionic surfactant.

The method for determining the sulfonic acid type anionic surfactant in the electrolytic solution includes a process (1) for treating heavy metal ions contained in the copper electrolytic solution or plating solution with a complex-forming agent, a processing (2) for forming a methylene blue aqueous solution in order to drive a compound soluble in an organic solvent from the water soluble sulfonic acid type anionic surfactant contained in the solution, a process (3) for bringing the treated solution into contact with the methylene blue aqueous solution to form an aqueous solution of a methylene blue complex of the sulfonic acid type anionic surfactant, a process (4) for bringing an aqueous solution of the complex into contact with an organic solvent for forming an extract, a process (5) for measuring the absorbancy of the extract at a measuring wavelength of 650 nm by an ultraviolet spectrophotometer, and a process (6) for inserting the measured absorbancy in the correlation of the sulfonic acid type anionic surfactant and absorbancy to calculate the amount of the sulfonic acid type anionic surfactant.